Serialization extension.

[amcgregor]

An extension to support the automatic serialization of basic lists and mappings returned by endpoints. Should support JSON and YAML (the latter only if the yaml support library is installed) using content negotiation to decide which format to use.

No matching format should probably cascade out of the view registered by the extension to allow other views to give it a try. (Or make that configurable, i.e. strict.)

Include tests.

[amcgregor]

Configurable default is in, customizable return type handling is in, and plugin use is in.