Bugfix/validate called twice

#21 
- only call Form instance validate from FormHandler if no FormHandler.errors is false
- validate is not called twice or unnecessarily
- bumped version
- docs update
- changelog update
![](http://24.media.tumblr.com/5da5862792daf8edcb74891cb4556c6a/tumblr_mldbe4xiLM1rmbz46o1_500.gif)