Relax bytestring >= 0.10 requirements adding a tiny builder like interface.

hit only use string7 and byteString from the builder module, making the
compat layer really tiny.

fix #9
bump version to 0.5.3
update indentation
Diff: add functions

sometime it's easier to get a diff from a ref and most of the time the diff is
performed between two references. So, it updated the getDiffWith to use Ref
instead of Revision. Added a function to get the Diff but from a revision.