Gettext localization

I'm basically finished with localization.
What is missing is
- [x] about/contact on main page. Should I localize this?
- [x] server errors
- [x] bike sharing I can leave this to @aguiraf before merging. Since he already did it in a way.
- [x] other languages (Will add after I solve previous tasks)
- [ ] Java properties also in po files
- [x] Totally broken GUI when translation strings are missing

Should I translate errors on a server or just add server error strings to client and choose translation based on error code?

I can also add Java properties files which are now used for path translations (bike path, cycle path, corner of x and y etc..) and server error translations to main POT file. And they could be translated same as everything else, but still available in properties files.

I also added [HOWTO](https://github.com/buma/OpenTripPlanner/blob/po_localization/README_LOCALIZATION.md).