postgis support

there's still a decent amount of work to be done on this:

1. the package naming and such for OSes other than ubuntu needs to be checked. I only run ubuntu so I haven't checked it against other things. I'm willing to do this, but I will need more time to do it as I need to grab vagrant boxes and whatnot.
2. the tests don't currently cover what happens when you specify the postgis version. I've tested it locally and it does what I expect, but not in any automated fashion, especially because ubuntu only supplies one version of postgis, and using manage_package_repo is currently broken as reported in #276

If someone wants to help me finish this for other OSes that would be great, or just let me know if I should do that and add some more commits or what.

Thanks!