Use memmem, strdup, and strndup from FreeBSD rather than libiberty

Also moved this to its own file
Update to the latest libtap

This is GPLv2+, so it's now compatible with our Apache license. The resulting
binaries would be licensed as GPLv3, but the only binaries this produces are
the test file binaries, so we don't care too much what the license is there.

The actual libmaxminddb.so remains Apache 2.0.
Made t/libtap a submodule instead of copying the files into our repo
Update submodule for travis build
Use git protocol URL for submodule

This seems to be needed for travis
Merge pull request #15 from maxmind/dave/remove-gpl-code

Dave/remove gpl code