Changed default_method to "temporary"

[aramk]

This fixes #15. I figured out what it was. I'm generating a lot of session variables with randomized UUID names. This is fine since on refresh they're cleared and have no impact. The issue is that persistent-session has the following line:

var default_method = 'persistent';

According to the docs, this should be set to temporary. At the moment, all calls to Session.set() will be storing the value in the persistent session which I believe is unintended.

[pauldowman]

Thanks! This also fixes #14. Note to @rgould to update the major version number since it's a breaking change.