Add export link for vendor list

We had a request for a .CSV of our vendor list. This feature enables
admin users to export it.
Add vendor/review/vegan_dish m2m validation

Fixes #155 on github.

When deleting a vegan_dish, the framework/db won't prevent you from
nulling out the best_vegan_dish on a review because few of these
fields are required. This commit adds a signal receiver that will
intercept changes to the m2m relationship of vendor<->vegan_dish. If a
vendor has reviews with non-null best_vegan_dishes, an IntegrityError
is raised. Extra validation was added to prevent the dreaded clear()
method as well.
Add terminal utils and linting to fabfile
remove stray print statements
Add default email configs to settings_local

This prevents all kinds of automation problems.
Merge pull request #261 from steventlamb/topic/admin_vendor_export

Add export link for vendor list
Merge pull request #262 from steventlamb/topic/add_m2m_validation

Add vendor/review/vegan_dish m2m validation
Merge pull request #263 from steventlamb/topic/dev_tools_and_cleanup

Topic/dev tools and cleanup