Added a WriteBytes function (WriteMem)

WriteBytes is a wrapper around WriteMem that lets you use
leptonica to change formats without persisting to disk.

I am not 100% certain I got this one right, but I gave it a shot.
Now return []byte not *[]byte
Renamed to "EncodedBytes" which makes more sense.
Merge pull request #4 from robertmeta/master

Added a WriteBytes function (WriteMem)