Move that little combo function inside get_converter
Put it all into one file

This makes it a pleasure to embed this into another library since only
one module needs to be renamed. Embeddability seems more important than
a package structure.

I might add some version statements for the ability to reduce bloat.
Comments
Make register immutable
Summarize generic API at top of file
Remove some redundant imports
Rename bpp -> bytes_pp (bpp means bits per pixel!)
Whitespace