Include a MPS writer

[pchtsp]

In PuLP we have python code to write mps files. Since we're in the process to integrate pysmps to read these files, it only makes sense to also migrate the writing of the mps files here.

This way, other applications could benefit from this code and it would be more likely that it would be compatible with other systems.

Ideally, I would also like to move the writing of LP files (and potentially adding reading support) but that would probably involve a package name change.

More information on how we do MPS writing:

https://github.com/coin-or/pulp/blob/master/pulp/mps_lp.py#L189

[jmaerte]

I will definitely look into this. First I will implement the test cases from your other issue.