Added failing unit test.

Test demonstrates that, unless time moves in exact 10ms steps, the
timer store can lose track of which bucket a timer is in, breaking
update functionality.
Force time to move in 10ms steps
Better comments to explain the bucket finding algorithm
UT for time moving to near one second boundaries
Merge pull request #27 from Metaswitch/fix-use-after-delete

Reviewer Ellie - Fix use after delete