Added the LDAP KeyProvider implementation

Copied the LDAP key provider from ia-common, no test cases, but hey, it works in production :)

I also opted for not including the python-ldap dependency in requirements.txt since it's not needed for basic functionality, and getting it to compile on OSX is kind of tricky. I guess the people actually using LDAP will know what to do (also, information about that is in the README)