Import i18n system into 6-4 branch

This request is to push i18n support in Mapserver (http://sphinx-doc.org/latest/intl.html)

I think it's more easy to maintain simple strings, the i18n seems for me better than the previous way of doing (create one dir for each language). It's more powerful to compare contents changes between original en version and another language

As I don't want to reinvent the wheel, I've modified the Makefile to deal with both "old" system and new one. So people happy with old system, keep it this way and other can migrate to i18n.

For english, nothing is changed in the process to make and update the doc

In others PR, I will submit an how to about the process for documentation translation (old and new way) and po files for French. 
I hope we can work behind the scene for the French translation and the deploy scripts and after some others tests, we will be able to push it in production.