Move _snappy out of global namespace

[Jenselme]

During the package review for fedora, the reviewer spotted that _snappy is in the global namespace. Since it is private it may be a good idea to move it in its own namespace. See https://bugzilla.redhat.com/show_bug.cgi?id=1244514#c10

[keszybz]

"its own namespace" → e.g. snappy._snappy.

[djsutherland]

This was fixed in #54.