Field Generator/Containment Field Changes [MAP] (Fixes #205)

Made containment fields shock you when you walk into them, not when you walk near them. Fixes #205.

Field generators, that are active, will now shock you; they did before but it was because of the containment field near it.
Changed the type path of the field generator and containment field:

>/obj/machinery/field_generator -> /obj/machinery/field/generator
>/obj/machinery/containment_field -> /obj/machinery/field/containment

Changed the map to have the new type paths.
The containment field will now shock you when it is created.
The containment field layer is a little above objects, so objects going through are under the field icon.
Cleaned up a bit of the code.