Added bzip2 formula

Implementation of bzip2 formula including tests. Passes audit and works on Mac OSX 10.9.4 with latest brew.
test can be simplified to:

```rb
testfilepath = testpath + "sample_in.txt"
zipfilepath = testpath + "sample_in.txt.bz2"

testfilepath.write "TEST CONTENT"

system "#{bin}/bzip2", testfilepath
system "#{bin}/bunzip2", zipfilepath

assert_equal "TEST CONTENT", testfilepath.read
```