Rename base package tessera => tessera_client

This is hopefully a temporary change, but having the tessera server
package extend the `tessera` module using `pkgutils` is making our
deployment process unhappy. For now, we’re giving the client package a
separate root module. I’m hoping the issues can be worked out before
1.0 to keep the module names consistent.
Merge pull request #1 from urbanairship/packaging-sucks

Rename base package tessera => tessera_client