Nano support for extensions and unknown fields.

You can use the processor option store_unknown_fields to switch
this support on:

  aprotoc --javanano_out=store_unknown_fields=true:/tmp/out

A separate option for extensions isn't required. Support
for unknown fields must be turned on to allow storing and
retrieving extensions, because they are just stored as
unknown fields. If unknown fields are switched on, extension
related code will be generated when a proto message includes
an extension range, or an extension is encountered.

By default, store_unknown_fields is false. No additional
code is generated, and the generator will error out if protos
contain extension ranges or extensions.

Change-Id: I1e034c9e8f3305612953f72438189a7da6ed2167