Timestamp skip: optional feature to skip unmodified source files to speed up builds

[danigiri]

When doing several maven runs, processing several JS files that haven't been modified is quite slow.
With this fork, setting the optional parameter checkFileModificationTimes=true means that maven will skip checking files that haven't been modified on the previous builds.
Features:

• 'mvn clean' will ensure all of them are checked on the next run, handy for CI builds
• skipped files are not present on the reports
• added or removed files between builds should be taken into account automagically
• all tests pass and new tests have been added to account for new functionality
• no API changes are needed and class structure is pretty much as it was before
• optionality means existing POM files are not broken and that this optimized behavior is an opt-in feature
• report formats are unchanged

Caveats:

• files with issues that haven't been modified will be skipped

Please let me know if you think this is useful and if there are any required tweaks, thanks.

[danigiri]

First commit shows some old code on one of the files (tried a crappy scheme for the timestamps report) but it should be OK if you take all commits.

dani:jslint4java-maven-plugin/ (timestamp-skip) $ git status                                                                                                                                       [1:33:27]
# On branch timestamp-skip
nothing to commit (working directory clean)
dani:jslint4java-maven-plugin/ (timestamp-skip) $ mvn -Pdist clean verify install -q                                                                                                               [1:48:54]
[debug] execute contextualize
[debug] execute contextualize

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running com.googlecode.jslint4java.maven.FileListerTest
Tests run: 6, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.216 sec
Running com.googlecode.jslint4java.maven.FileTimestampExcludeListerTest
Tests run: 4, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 2.043 sec
Running com.googlecode.jslint4java.maven.JSLintMojoTest
Tests run: 14, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 5.797 sec
Running com.googlecode.jslint4java.maven.MultiReportWriterTest
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.006 sec
Running com.googlecode.jslint4java.maven.ReportWriterImplTest
Tests run: 4, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.009 sec
Running com.googlecode.jslint4java.maven.TimestampFormatterTest
Tests run: 3, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0 sec

Results :

Tests run: 32, Failures: 0, Errors: 0, Skipped: 0

[debug] execute contextualize
[debug] execute contextualize
dani:jslint4java-maven-plugin/ (timestamp-skip) $