Implement the support to t:md5 into libmodsecurity

[zimmerle]

Transformation md5 needs to be implemented inside libmodsecurity.

The documentation of the transformation can be found here:
https://github.com/SpiderLabs/ModSecurity/wiki/Reference-Manual#md5

The file already exist in the repository:
https://github.com/SpiderLabs/ModSecurity/blob/libmodsecurity/src/actions/transformations/md5.cc

[zimmerle]

Implemented at 7ccf54d