Add MockSession class to test module

Useful for running test requests that trigger session activity but the
test case is not interested in what is get or set in the session.
Add support for multiple/non-local tile hosts

This commit removes the requirement that tiles need to be served from
the same host as the application and allows specifying multiple tile
server hosts to allow browsers to request more tiles simultaneously.
Merge pull request #312 from jwalgran/topic/multiple-tile-hosts

Add support for multiple/non-local tile hosts