Gradle build system

I've added a gradle build script, which simplifies dependency management. The lib folder now contains only those jars that I cannot easily replace with versions available through the [Central Repository](http://search.maven.org/).

To build with this, get [Gradle](http://www.gradle.org/) and do:

    gradle build

To recreate the eclipse project and classpath files, do:

    gradle eclipse

To run the test suite, do:

    gradle test

I've noticed all but 3 of the 180 tests it runs succeed. Those 3 failing test cases all seem to involve image comparisons: Approvals incorrectly thinks that two images (approved vs. received) differ when they do not.

This pull request also incorporates commits from other pull requests. The changes @gamlerhart made improved the number of tests that pass, and Mario's changes seems useful for working with Maven build systems.