Add CreateInOrder command.

Created a CreateInOrder command that will use a directory as a queue. It
uses the POST property of etcd to create ordered keys within the
directory; each key is guaranteed to be greater than the key that
preceded it.
Change dir test deletes to a variable.

The dir that updates in set_update_create_test.go's TestCreateInOrder
function are posted to is already put into a variable, so change the
defer to use that variable to ensure the dir is deleted at the end of
the test.
Merge pull request #105 from paddyforan/create_in_order

Add CreateInOrder command.