Please add libbcrypt

This formula builds libbcrypt.dylib from [crypt_blowfish](http://openwall.com/crypt/) and adds it to /usr/local/lib.
Can a test be added to do something more substantial than e.g. `--version` or `--help`? See `cmake.rb` for an example of an application formula with a good test and `tinyxml2.rb` for an example of a library formula with a good test. Thanks!