LEMON: C++ Graph Library

The LEMON library is used to model and optimize graphs. There's a formula for
lemon, the parser generator, so I thought it'd be best to add 'graph' to this
formula's name.

In any case, this one is building correctly as of OSX 10.8.3. There's only
one problem, though: I had to use the '--disable-tools' flag because, for some
reason, there were problems when building them. I'm not entirely sure why, but
I'll continue to investigate.

Nonetheless, I think that this formula is good enough to be merged -- my programs that use it are working fine and the brewing process is working without problems. :)