Support different transifex configurations

I was thinking about arrays and things and in the end I found that there's just too much code to change without knowing where it would have any impacts so I just introduced an optional option for the transifex config param. Default is "transifex" and would thus still work but by using "-t mykey" you can force the toolbox to take the "mkey" instead of "transifex" :)

I was wondering wether we should force to use "transifex-" as prefix...