Added 'Clear Text' keyword for clearing the text value of a text entry element.
Added test for new 'Clear Text' keyword.
Renamed new 'Clear Text' keyword to 'Clear Element Text'

Renamed keyword to match corresponding and already existing keyword in the
Selenium2Library Java port. Update acceptance tests and removed invalid tests.
Removed unnecessary check on return value from _element_find.
Merge pull request #297 from emanlove/clear-text

Added new keyword 'Clear Element Text'