Added key to Provider class
Added validate method to Provider
Merge pull request #353 from psav/fix_cloud_prov_tests

Added validate method to Provider
Added select() function

Added select() to allow user to specify the select type (value/text) as
a tuple and then act accordingly.
Merge pull request #361 from psav/add_select

Added select() function
Fixed stale element when re-using trees
Merge pull request #362 from psav/fix_stale_chloroplast

Fixed stale element when re-using trees