Js.signals

Fixes

- allow `add()` and `addOnce()` same method multiple times if passing
different context. (39)

- Improve `dispatch()` performance if `Signal` doesn't have any listeners.

API changes

- Added:
- `Signal.memorize`. (29)
- `Signal.forget()`. (29)
- `Signal.has()`. (35)

Other

- changed the way the code is wrapped to have a single distribution file for
all the environments. (33)

Fixes

- improved `SignalBinding.detach()` behavior. (25)

API changes

- Added:
- `SignalBinding.prototype.isBound()` (25)
- `SignalBinding.params` (28)

- Removed:
- `SignalBinding.prototype.dispose()` (27)

Other

- minor code cleaning, better error messages.

Fixes

- removing a listener during dispatch was causing an error since listener was
undefined. (24 - thanks paullewis)

Other

- minor code cleaning.
- renamed distribution files to "signals.js" (22)

- added NPM package.json and CommmonJS wrapper for NPM distribution. (21 - thanks tomyan)