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.)

There was some rearranging of tests previously in test_vpc into different files, so no tests were actually deleted.