Add function http_parser_version().

Fixes #115.
Do not accept PUN/GEM methods as PUT/GET.

* Encountering them returns an error, `HPE_INVALID_METHOD`
* Tests have been added.
Further request method check strengthening.
test: add amazon.com response test
Update misleading comment.

The HTTP_MAX_HEADER_SIZE check is not there to guard against
buffer overflows, it's there to protect unwitting embedders
against denial-of-service attacks.
Add syntax highlighting to README C code
Use unsigned int as bitfield type.