Rails 3.1.0+ compatibility (and a few other minor improvements)

[Empact]

Hey there,

I made some improvements to spatial adapter to get it working under Rails 3.1.0 for my own use, and posted about fork here: #26

Several people have confirmed that it works for them and one has suggested a pull request, so here it is.

One issue, which I noted in my original comment to that thread:

I have a working fix, but still an outstanding issue or two. Most notably, the schema dumper doesn't seem to be picking up its extensions, when run from within my rails project. The schema dumper specs pass.

Also note I've only been testing on postgres.