Add / centralize degree <-> spatial resolution helpers.

ichnaea.models.DEGREE_DECIMAL_PLACES
ichnaea.models.DEGREE_SCALE_FACTOR
ichnaea.models.from_degrees
ichnaea.models.to_degrees
Add CellKey helpers to ichnaea.tasks, tidy up a bit.
Fix two typos in comments.
Note which keys are expected to be moving in testcase.
Add ichnaea.models.CellBlacklist.
Implement moving-cell blacklisting, close #134.
Merge pull request #144 from graydon/bug-134-add-cell-blacklist

Bug 134 add cell blacklist