Implement the support to t:base64Decode into libmodsecurity

[zimmerle]

Transformation base64Decode needs to be implemented inside libmodsecurity.

The documentation of the transformation can be found here:
https://github.com/SpiderLabs/ModSecurity/wiki/Reference-Manual#base64Decode

The file already exist in the repository:
https://github.com/SpiderLabs/ModSecurity/blob/libmodsecurity/src/actions/transformations/base64_decode.cc

[zimmerle]

Implemented at: e48f468.