Add numerous functions from 3-4 forks (add lat and lon live textfields, and map resizing in config. Smaller bugfixes etc)

As a response to a request in https://github.com/beyondthestory/rails_admin_map_field/pull/7 - this is about 30 commits made by Jules Laplace, vala, Jason Langenauer, Bartlomiej Skwira all rebased/squashed. Adds stuff like:
- adding fields in lib
- geocode the address and use that to set the lat/lng
- make using country and state fields optional
- add map_width and map_height, and run map code in gmap load callback
- lat and lon live textfields 
- map resizing in config
- smaller bugfixes


Rebase against current master was too tricky for me, there were too many changes for me to merge it correctly. Merge at your own risk :P