Change extract to unpack

[Teteros]

Arguably more intuitive as unpack is the direct opposite to pack

Closes #4

[phrohdoh]

easage_extract.rs will need to be renamed to easage_unpack.rs and this line should probably be updated.

As-is this will fail to compile because the easage_unpack module won't be found (it doesn't exist with that name).

[Teteros]

Good catch, I forgot to search/replace for capitalised Extract.