Add comment to find_references_by_field

With link to API documentation.
Add find_references method to Podio::Item

Using preferred block syntax for specifying url. No options hash, as
there are no other options as of now.
Added date election support.
flesh out DateElection a bit more
voting needs a value now
expose the value property
Added methods to update description and usage directly
Add config to item diff model
Method to count total number of spaces
Add capability names to account providers
Get conversations list
Added some more properties on the conversation.
Added ConversationEvent model
Added options to conversation find_all call.
Added type attribute to Conversation model
Added friend-model
Add missing properties
More properties to add
Added support for variable references in has_one to support operations where a data hash can return several different models
Let data on conversation event by automatically mapped to the correct class depending on the action