Update angular-timer.js

Issue found:
autoStart attribute does not properly work on the directive.
1) autoStart does not exists: Timer starts automatically (expected behaviour).
2) autoStart exists, set to fasle: Timer starts automatically (expected behaviour).
2) autoStart exists, set to true: Timer does not start automatically (NOT expected behaviour).

This happens because variable $scope.autoStart (on line 256 in the "if" statement) is a 'string' type.
So "$scope.autoStart === true" never passes.

Proposal solution will fix the problem with variable type.