Support for new mechs (GSSAPI, GSS-SPNEGO) and secure layer (GSSAPI, DIGEST-MD5)

This pull adds new features:
* Mechanisms
  - GSSAPI (kerberos v5) using gem gssapi (which uses native library by
    FFI module, with support for windows/linux/darwin). 
  - GSS-SPNEGO using gem rubyntlm
* Secure Layer (similar to SSL)
  - Implemented for GSSAPI using gem gssapi
  - Implemented for DIGEST-MD5 using OpenSSL crypto
    + support auth-int
    + support for auth-conf (ciphers rc4, rc4-40, rc4-56, des, 3des)
* Added some more tests for new featues
* Updated tests for rspec 2.x