Add gzip support for s3 uploading.
Add tests for MultipartWriter and GzipMultipartWriter.
Simplify writer interface and do away with pusher entirely.
Add proper IMultipartWriter interface.
Check part lengths produced by GzipMultipartWriter.
Merge branch 'develop' into feature/issue-1085-gzip-s3-upload-support
Add tests for three-part and zero-part gzipped files.
Don't assert on specific part lengths because that seems to vary with gzip version.
Speed up large data test by a factor of 30.
Merge branch 'feature/issue-1085-gzip-s3-upload-support' into develop