Catch strptime format error

I've add these enhancement in this PR.

* catch strptime format error which caused by "Custom Time Format"
* add default time format examples for apache and syslog
* add link for ruby strptime document
* use more "font awesome" in form panel (asterisk, hummer, clock etc...)

![screenshot](https://cloud.githubusercontent.com/assets/1734549/3183449/1ca3567a-ec6b-11e3-98f4-6dcca51374f1.png)