Replace SRILM with KenLM

[lefterav]

KenLM can already estimate models in a quite feasible way. Second advantage is that it is open source and we can include it in the package or download it with the installation script. It shouldn't be too difficult to just change the commands, cause of the same (ARPA) n-gram file format.

[lspecia]

Good motivation for KenLM: we can distribute it, so people can generate the LMs from the parallel corpora as opposed to having to provide them.