Return error message if group_by is incorrect

@pbadenski
Names starting with "$" are reserved keywords in Mongo

@pbadenski
Filtering should be disabled for internal fields

@pbadenski
Allow optional trailing slash

@gtrogers
@pbadenski
Revert "Allow optional trailing slash"

This reverts commit 2819e5365731de8428d224252bc16e92785e3438.

This doesn't work with how we're rewriting the URL. The client gets a
Location header pointing to the internal host name of the app.
Fix package versions in requirements.
Merge pull request #19 from alphagov/fix-dependency-versions

Fix package versions in requirements.
Add coverage reporting
Add main module qualifier to coverage generation

Python 2.6 does not automatically run the __main__ module of a package
so it has to be provided explicitly.
Add coverage related files to .gitignore
Write pep8 output to file.
Use color output for behave
Merge pull request #20 from alphagov/coverage-reporting

Add coverage reporting
Merge pull request #15 from alphagov/extend_validation

Extend validation
Merge branch 'master' into sort_and_limit

Conflicts:
	backdrop/read/validation.py