Fix csrf validation without time limit (time_limit=False)

With time_limit=False, generate_csrf will generate a token with an empty
'expires' value. The validate_csrf function will always fail because it
tries to parse an empty string as float. We should parse this value only
if we care about the time_limit.
Merge pull request #129 from BenoitZugmeyer/non-expiring-csrf

Fix csrf validation without time limit (time_limit=False)
It's time to support wtforms 2.0

https://github.com/lepture/flask-wtf/issues/94
Get default time limit from configuration for Form.

Related issue: https://github.com/lepture/flask-wtf/issues/131
Travis for WTForms 2.0.1