Simplify wait method assertions

 - The wait for invisibility method is particularly problematic if you
   have bad luck, the element could disappear between the
   element_exists? and find_first().visible? test, giving you a
   NoMethodError
Fix typo
Merge pull request #28 from excepttheweasel/master

Simplify wait methods
added no_element_exists method and tests
Merge pull request #29 from johnwake/master

Tests for no_element_exists
updating history and todo