rhbz888090 - Add common utilities for ETag processing.

Add a Digest Writer which keeps a digest for all written bytes.
Add a FileDetails object that contains details about a file.
PoWriter2 now returns one of these FileDetails objects whenever it writes a file.
rhbz888090 - Add utility method to calculate a file's MD5 checksum.
Merge branch 'rhbz888090' into integration/master

Conflicts:
	zanata-adapter-po/src/main/java/org/zanata/adapter/po/PoWriter2.java