#43 MD5: patch to fix GCC warning

[Tristan Matthews]

Currently GCC (4.8) reports the following warning for when compiling md5.c in libiax:
"dereferencing type-punned pointer will break strict-aliasing rules"

The attached patch resolves the issue by using a union in MD5Context for the "in" member, so it can be accessed as either a uint8_t[64] or a uint32_t[16].