Unit tests for VPC, support DetachVpnGateway + minor VPC functionality

This pull request adds unit tests for all methods in the VPC module not covered by pull request #1816. It also updates a few methods to expose functionality not currently exposed.

* replace_route_table_association now returns the new association ID (previously only returned “true” for success.)
* create_route now supports interface_id, just like replace_route does
* The bgp_asn argument to create_customer_gateway now is documented to take an int instead of str, like the AWS documentation says. Also updated the CustomerGateway class to parse the int.
* Adds detach_vpn_gateway, as someone on the boto-users mailing list noticed was missing

PEP8 fixes were applied throughout the VPC module; the only PEP8 violations now are line length violations (my company uses 100 char long line, I didn’t bother going back and shortening it to 80 chars long.)

Most test XML came directly from the online API docs provided by Amazon, with minor fixes for typos or to make IDs match. 1 or 2 tests added some XML to the examples to be able to test functionality not exercised by the example.

There was some rearranging of tests previously in test_vpc into different files, so no tests were actually deleted.