Fixing an issue with browser disconnection and task timeouts.

This commit fixes the following bug: when a browser is disconnected
during a test, the task timeout function is not canceled. When it is
executed, it assigns a new task to the disconnected browser, which
will lead to a timeout (as there is no browser connected to execute
the task), and then another task will be assigned and so on...
Better error msg when test case has syntax error

Close #64.
Filter tests to be run by jasmine

Possibility to filter which attester specs should run;
useful for debugging attester.

Usage:
 grunt jasmine_node --grep=<pattern>

Close #60.