Random: include the Crypt_* classes if they're available

also error out if none are available
SSH2: replace _is_includable() with stream_resolve_include_path
Random: add explanatory comment
Replace stream_resolve_include_path with phpseclib_is_includable.
Random: a few changes to the stream_resolve_include_path_function
Drop Net_SSH2::_is_includable() method.
phpseclib_is_includable() -> phpseclib_resolve_include_path()
Merge pull request #385 from bantu/a-random-change

Random: include the Crypt_* classes if they're available

* bantu/a-random-change:
  phpseclib_is_includable() -> phpseclib_resolve_include_path()
  Drop Net_SSH2::_is_includable() method.
  Random: a few changes to the stream_resolve_include_path_function
  Replace stream_resolve_include_path with phpseclib_is_includable.
  Random: add explanatory comment
  SSH2: replace _is_includable() with stream_resolve_include_path
  Random: include the Crypt_* classes if they're available