Initial subnets code.

Taken from:
http://bazaar.launchpad.net/~goamz/goamz/trunk/view/49/ec2/subnets_test.go
http://bazaar.launchpad.net/~goamz/goamz/trunk/view/49/ec2/subnets.go
More merging from http://bazaar.launchpad.net/~goamz/goamz/trunk/files/49/ec2

* Added CreateVPC, DeleteVPC, VPCs (describes vpcs) to vpc.go along with accompanying tests, responses and updates to ec2test/server
* makeParamsVPC() -> makeParams()
* errorCode(err) -> err.Error()
* c.Check -> c.Assert (and updated tests to use gocheck)

Tests pass!
Merge pull request #3 from czos/add_subnets

Add subnets