Add the cow_multipart module for multipart message parsing and building

This is a new implementation that has been measured to be up
to 33% faster than the implementation that was in Cowboy.
Use the inline macro for cow_http:parse_fullhost

Tidying the code a little.
Test cowlib on all releases since R15B, plus maint and master

It passes on everything!
Update cowlib to 0.5.0

Added support for multipart building and parsing.
Added an include file for doing inline lowercase of binary strings.