Implemented additional subnet attributes

This change implements name, dns_nameservers, gateway_ip, host_routes subnet
attributes. Also, disables allocation_pools and enable_dhcp subnet attributes.

*    Modified Subnet model, added DNSNameserver model (WARNING: DB changes!)
     -    Added name, enable_dhcp, and dns_nameservers to Subnet model
     -    Added subnet_update, route_update, dns_create, dns_delete
*    Implemented api extension to disable allocation_pools and enable_dhcp
*    Implemented gateway_ip, dns_nameservers, host_routes for create_subnet
*    Implemented gateway_ip, dns_nameservers, host_routes for update_subnet
*    Fixed broken subnet-related tests
*    Implemented update_subnet tests
*    Implemented create_subnet tests
*    Integration tested with quantum+quark+sqlite
Moved implementation of PUT /ports to POST /ports
Added coverage for api_extensions_path configuration
Merge pull request #43 from asadoughi/put_to_post_ports

Moved implementation of PUT /ports to POST /ports
Merge pull request #51 from asadoughi/update_subnet_squash

Implemented additional subnet attributes
Merge pull request #54 from asadoughi/extensions_coverage

Added coverage for api_extensions_path configuration