Allow iodized to override on a per session basis

Can be overridden to be true or false. Passing nil will clear the override.

Usage:


``` ruby
# Set the feature true for this session
Iodized. override_for_session("feature_name", true)

# Determine whether the feature has been overridden
Iodized.overriden_for_session?(feature)

# Clear the feature for this session
Iodized.override_for_session("feature_name", nil)
```