It now receives notifications about started/finished builds/steps via
PostgreSQL. This gets rid of the (substantial) overhead of starting
hydra-notify for every event. It also allows other programs (even on
other machines) to listen to Hydra notifications.

* 'eval_started' has the format '<tmpId>\t<project>\t<jobset>'.

* 'eval_failed' has the format '<tmpId>'. (The cause of the error can
  be found in the database.)

* 'eval_added' has the format '<tmpId>:<evalId>'.

Plugins are now disabled at startup time unless there is some relevant
configuration in hydra.conf. This avoids hydra-notify having to do a
lot of redundant work (a lot of plugins did a lot of database queries
*before* deciding they were disabled).

Note: BitBucketStatus users will need to add 'enable_bitbucket_status
= 1' to hydra.conf.

In particular, doing a 'select * from Jobsets where ...' must be
avoided, because the 'errormsg' column can be very big.

Turn hydra-notify into a daemon