Add mercury checker using Melbourne Mercury Compiler

This patch adds the [mercury](http://mercurylang.org/) checker **mercury-mmc** based on the *errorcheck-only* mode of the Melbourne Mercury Compiler. It also adds a unit test for mercury-mmc.

Unfortunately, you have to compile mmc from source to get a working compiler / error checker and the compilation needs a lot of time (> 1h). So I have no idea how to add this to Travis CI.