libgdiplus new formula

According to [this](http://www.mono-project.com/docs/compiling-mono/parallel-mono-environments/#building-sources), at least, libgdiplus, gtk-sharp, gtk-sharp-2 and mono-tools should be part of Mono runtime.  

> it probably shouldn't be a separate formula.

So maybe a new PR to put all of libraries mentioned above into `mono.rb` using resources?