Added appendField function

#523

Allows user to add text to text already existing in a field, or select an additional option on a multi select box. Used for seleniumIDE functions (sendKeys, selectOption)
- Update to appendField function to handle checkbox value or label
- Removed delay from webdriver, all tests still passed
- Added tests for appendField Function
Merge pull request #631 from allmyitjason/patch-1

Added appendField function