Add new "checkstyle" target to the Quartz build.xml.

It will be run as a dependency of the "compile" target unless "skip.checkstyle" is defined, or the checkstyle.jar is not on the classpath. Checkstyle uses the "GNU LESSER GENERAL PUBLIC LICENSE Version 2.1" so for now it won't be added to the lib/build directory.

Below is the proposed checkstyle file. I think a lot more checks could be added without much debate, but I tried to keep it to just very basic formatting for the first cut. Perhaps the only one that is a bit of a stretch is that I enabled the "NeedBraces" check. This basically requires that single line control blocks are enclosed in braces:

if (doit)
   doit();

becomes:

if (doit) {
    doit();
}

Though this does add an extra line, my experience has been that about 1 in a hundred times this saves you from are a really hard to track down bug, and, in fact, I believe Quartz actually has at least one such bug (QUARTZ-340).

I'm also supporting a suppressions.xml file (which should hopefully be empty for now), so that when we do add more interesting checks we can exclude files that should be allowed to break the rules (I feel pretty strongly that checkstyle should never be allowed to force one to write hard to read code, better to turn it off in such cases. It should be a helper, not a dictator :)

PROPOSED CHECKS:

<!-- Checks for blocks. -->
<!-- See http://checkstyle.sourceforge.net/config_blocks.html -->
<module name="NeedBraces"/>
<module name="LeftCurly">
<property name="option" value="eol"/>
</module>
<module name="RightCurly">
<property name="option" value="same"/>
</module>

<!-- Checks for imports. -->
<!-- http://checkstyle.sourceforge.net/config_import.html -->
<module name="IllegalImport"/>
<module name="AvoidStarImport"/>
<module name="UnusedImports"/>

<!-- Checks for whitespace. -->
<!-- http://checkstyle.sourceforge.net/config_whitespace.html -->
<module name="TabCharacter"/>

<!-- Checks for Modifiers -->
<!-- See http://checkstyle.sf.net/config_modifiers.html -->
<module name="RedundantModifier"/>
<module name="ModifierOrder"/>

<!-- Misc. Checks -->
<!-- See http://checkstyle.sourceforge.net/config_misc.html -->
<module name="Indentation"/>

<!-- Class Design Checks -->
<!-- See http://checkstyle.sourceforge.net/config_design.html -->
<module name="HideUtilityClassConstructor"/>