Remove libmagic dependency in favor of builtin binary detection

The builtin binary detection is likely less accurate than libmagic
in some ways, but will be more accurate in the more common cases.
remove libmagic source and patch
add another test case
scan the entire buffer for null bytes
only scan the first 10MB
make binary scan length configurable at runtime
update docs
add some more binary content types to sniffing
make png sniff a little more readable
add some more test fixtures
fix typo in comment
add some more fixtures to test UTF-16|32
Merge pull request #65 from brianmario/remove-libmagic

Remove libmagic dependency in favor of builtin binary detection