Avoid extra virtualenv on travis, try to get it to use the right Python
try to trick pip into installing argparse
use unittest2 under Python 2.6 to get nicer test assertions
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.