Add option for excluding high-level pageload tests

These tests are slow, do not need to be run frequently, and artificially
inflate code coverage statistics.

Add a separate fabric task for test and test_all

remove PageLoadTest from code coverage measurement
Add hooks for development requirements

make settings and settings_local support dynamically adding development
apps to the stack, separate from runtime apps. Move development
requirements to a separate pip requirements file.
Add debug_toolbar to development environment
Remove duplicate 'vegancity' in INSTALLED_APPS
Make travis use pip mirrors