Support "mvn test"

- Lets maven run OpenTSDB unit tests.
Modifies pom.xml.in to configure the surefire plugin properly. Also creates
plugin_test.jar that some unit tests depend on.

- Makes it easy to use Eclipse.
The steps for OSX will be:
   0. Install OSX autoconf
     port install automake autoconf libtool
   1. Generate pom.xml and the Eclipse-friendly directory structure overlay
     ./build.sh pom.xml
   2. Build the maven project
     mvn clean compile
   3. Import the Maven project into Eclipse.
   4. Include target/generated-sources in Eclipse source path.

- Added some file extensions and working files to .gitignore.

NOTE: Submitted on behalf of third-party contributors:
 * Guenther Schmuelling <schmuell@pepperdata.com>
 * Kimoon Kim <kimoon@pepperdata.com>