dont include tests in coverage calculation

use basetestcase for most test cases that require predefined user

fix docs page crashing if welcome code is expired

fix tests and optimize authentication and fix some unclosed markup

Otherwise, if period=1m and grace=1m then we can end up generating huge useless tables.

Check.n_pings now stores the total number of pings the check has ever received. Running "prunepings" command doesn't affect this field. +a new "prunepingsslow" command which works in smaller chunks so is appropriate for initial pruning of a huge api_ping table.

Update README.md to reflect that django1.9 is used

check token need to handle invalid user & invalid token correctly