Rewrite std_dev_across_tracts

- Now takes a list of target columns to be summed
- Renamed, cleanup, PEP8
Generator for returning real and fake boundaries
Add metric detail view, alter score view

- Metric detail is specified by the boundary and the node slug.
- Score view drops citations, changes references to boundaries,
  and adds URI to detail view
- New boundary view for fake boundaries
- Algorithms refactored to allow scoring by location or boundary
- Census algorithms refactored for common code

Also
Drop ScoreMetric fields

These fields make dumpdata / loaddata harder, but are unused in the
code and not used for documentation.  Use params until we have a
better idea what the score model will be.
Merge pull request #21 from jwhitlock/metric_refactor

Metric refactor