Correct the order of suggested syntax for extension imports

According to https://github.com/mitsuhiko/flask/issues/1092#issuecomment-47118613 and https://github.com/mitsuhiko/flask/pull/1085#issuecomment-45466907 , the correct order to attempt to import extensions should be flask_foo, then flask.ext.foo, then flaskext_foo.
Merge pull request #1218 from jeffwidman/patch-1

Correct the order of suggested syntax for extension imports