Added option argument for Item.find_by_filter_values
Added new ItemTransaction model
Added support for retrieving a single item transaction
Fix autoload error

See https://github.com/podio/podio-rb/issues/16
Remove date election
Remove this also
Extension model
Merge pull request #14 from pjmuller/options_item_find_by_filter_values

Added option argument for Item.find_by_filter_values
Add remaining extension operations
Get user extensions is for current user only
Add user relation and status property
status is state and add extension_id
Added method to get single experiment to Experiment model
Added delete variant method to Experiment model
Add missing properties
state is now status
Added has_many relationship between Promotion and ConditionSet
Add more extension properties
...and some more properties I missed
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