Models: Refactor some common code between tool shed and Galaxy into galaxy.model.orm.
Models: Refactor common code out between Galaxy and Tool Shed related to create_engine.
Introduce new shared class for app.model.

Shared across Tool Shed and Galaxy to reduce code duplication. Previously this was just a bunch, making it a real class allows placing some shared logic in there.

This refactoring also eliminates some global variables galaxy.model.mapping.Session, galaxy.model.mapping.context, and same for tool shed. This may break things, but we should figure out what these things are and fix them.
PEP-8 fixes for mapping tests.
Get rid of old get_suite in mapping.py. Unit test code moved to tests/unit.
Unify 'now' from datetime and associated comment across galaxy and toolshed.
Models: Refactor tool shed installation related models and mapping information in Galaxy's models into new module galaxy.model.tool_shed_install.

This is work toward being able to store tool shed installation data into a second database. There is still a lot TODO, all access to these models are still through galaxy.model and the existing ModelMapping object, these references need to be refactored.
Replace references to galaxy.model.ToolShedRepository with new galaxy.model.tool_shed_install path.
Model: Introduce app.install_model.

All references to galaxy.model.tool_shed_install models should go through this instance. Right now it is a reference to the same ModelMapping instance, but the goal is to allow them to be two different mappings.