steam-crypt: implemented cryptographic library wrapper

Currently, the plugin handles some cryptographic operations internally
via GMP. This was done initially to prevent from having to implement
and maintain three different cryptographic libraries. However, as time
has progressed, it is has become evident that this is not practical.

The cryptographic library wrapper supports GnuTLS, NSS, and OpenSSL,
which are the three libraries supported by bitlbee. As a result of
making use of these libraries, GMP is no longer needed.