Add hooks to the encoder to allow encoding tags

[mildred]

There is a few things here:

• If a type implements a marshaller interface, use this interface to marshall the value
• Make the tag type implement the marshaller interface
• Export Encoding functions to allow access encoding small data from outside of this package

This is necessary to encode IPLD using tags as defined in ipfs/specs#61. The last item is useful to implement the marshaller interface and to generate the expected byte slice in unit tests.

[mildred]

updated to the switch statement

[whyrusleeping]

alright cool, LGTM