Document available options in README.md
Keep consistency between touch and mouse
Add onBegin() & onEnd()
Merge branch 'event'

Conflicts:
	signature_pad.js
Use onBegin() & onEnd() for event hook
Add onBegin() & onEnd()
Update README.md about onBegin() and onEnd()
Merge branch 'event'

Conflicts:
	signature_pad.min.js