Refs #13 - external identifier is a string.

So we also need to sanitize it.
Target .NET 4.5 for Behavior and Client.
Refs #13 - automatically serialize object to json.

Introduced an attribute (JsonConvertible). A datacontract with
that attribute can be used by the Chiro.Civicrm.Behavior without
special configuration.
Refs #13 - saving a contact.

Saving works, but returning the results doesn't work as expected.