Factor out test utilities to separate sub-package
Add GetProjectReferences tests
Implement GetProjectReferences function
Refactor project resources extraction

* Add GetMathematicaResources function.
* Change GetExecutionBuildCommand to subvalue on Resources object.
* Add Resources object subvalues extracting Sources, Java, GUI,
Palettes, StyleSheets and IndexExcludes paths.
* Add/adapt tests.
Add project sources to $Path in test runs
Add testsExtraPath ant property

It's value should be a comma separated list of paths. Those paths are
added to Mathematica $Path in test runs.
Merge pull request #6 from jkuczm/multiple_versions_tests

Extra `$Path` for `runTests` target