- avoid comparing to null with ==

[srkimir]

When new breaker is created via new new CircuitBreaker({...}), self._forced is undefined (not null). Now when _executeCommand gets called there is:

if (self._forced == null) {
  self._updateState();
}

Which will be interpreted as if (undefined == null), which will be true, and library is on safe side. Now imagine that at sometime you will run eslint against this code (http://eslint.org/docs/rules/no-eq-null) and replace above check with if (self._forced === null) (which is anyway, safer than using just ==), whole library would be unusable, right? cause (undefined === null) will lead to false. Problem can easily be solved with if(!self._forced) as proposed in PR, cause it will return true if self._forced is undefined or null.

Other than that there is also proposal to use === instead of == in whole lib, and also using string constants such as OPEN, HALF_OPEN and CLOSED instead of 0, 1, 2, it improves readability, and if you decide to merge PR it is also must have, otherwise when self._forced is equal to 0, than
if (!self._forced) would be interpreted as true because of if(!0)

Tests results with proposed changes:

Running "jshint:all" (jshint) task
>> 2 files lint free.

Running "jasmine:all" (jasmine) task
Testing jasmine specs via phantom
..........................
26 specs in 0.011s.
>> 0 failures

Done, without errors.