Implements the maximize_window function

Specs documented here:
https://code.google.com/p/selenium/wiki/JsonWireProtocol#/session/:sessionId/window/:windowHandle/maximize
added test for maximize window
mock data for linux and Mac (don't own a mac, so blindly added)
Restore file permissions after #169
Verify that window size increases after maximize

For #169
Update os x mocks; linux & windows forthcoming

[skip ci]
Update version and changes; another trial release soon