Improve Bloom.proj

- Allow to use Bloom.proj on a developer machine
- Enable running unit tests from Bloom.proj also on a developer machine, not just from TC

NOTE: this change depends on libpalaso PR #156 (https://github.com/sillsdev/libpalaso/pull/156).
shouldn't AssemblyFile="$(MSBuildCommunityTasksPath)/MSBuild.Community.Tasks.dll" ?  ie, variable reference for the directory path?