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

[BartlomiejSkwira]

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