Rename _blob_data to blob_data

No reason I can see not to make blob_data consumable (indeed, if it
isn't, many tasks require poking at the dulwich repo directly), so
follow the naming convention to make it so.

Signed-off-by: Gregory M. Turner <gmt@be-evil.net>
Merge pull request #35 from cscorley/master

Filter refs on fetch for issue #33
Merge pull request #34 from gmt/master

Rename _blob_data to blob_data
Removes the dependency on the mimer library

Uses functionality built-in to dulwich to determine whether a file is
treated as binary or text. This allows Gittle to behave closer to what
`git diff` actually produces.
Changes deprecated revision_history to get_walker
Merge pull request #36 from cscorley/removemimer

Removes the dependency on the mimer library
Merge pull request #37 from cscorley/dulwich_deprecation

Changes deprecated revision_history to get_walker