major version bump

Bump major version as this breaks backwards compatibility.

I changed the code to support pg @ v1.0 and onward.  Made it a peerDependency so it wont install unless you have a proper version of pg installed.  This means removing all pauseDrain() and resumeDrain() stuff.  If you create a transaction object & pass it in a client, you are responsible for returning that client to the pool after the transaction is finished or disconnecting the client if the client is not pooled, etc...

I think what would be cool is to be able to pass in a pool instead of a client & the transaction will automatically check out a client from the pool and return the client to the pool upon `abort`, `commit`, or `rollback`

I changed the tests slightly to be more 'sync' by only exiting the afterEach when the client finished disconnecting.