Makefile: factor out SPARKR_VERSION to reduce potential copy&paste error; cp & rm called with -f in build/clean phase; .gitignore includes checkpoints and unit test log generated by run-tests.sh
Update Makefile (style uniformity)

Add a space for style uniformiity
Merge pull request #72 from edwardt/master

(minor improvement) Use variable for SparkR assembly versioning in Makefile; ignore folders and files generated in unit tests to avoid accidental checkins