MD5 implementation replacement & contrib/ removal

Half of this is the MD5 replacement as discussed in #120. The code is OpenSSL-compatible which is in turn RSA-compatible (even the function prototypes are similar) so this shouldn't functionally change the code. A simple test case seemed to produce the same output, it is however untested in a twemproxy environment.

The other part is the removal of the embedded yaml & contrib/. The commit message has all the reasoning behind this, let me know if you need additional clarifications on why I needed this.

(while the two may seem unrelated with each other, they're the only blockers I found on my way to packaging & inclusion in Debian main)