rlp: allow encoding non-empty interface values

Difficulty was broken when refactored.

This needs to be supported because []someInterface does occur sometimes.

Funny enough, the fix involves changes to the decoder. makeDecoder
cannot return an error for non-empty interfaces anymore because the type
cache builds both decoder and writer. Do the check at 'runtime' instead.

rlp: add functions for encoding

I'm reasonably confident that the encoding matches the output of
ethutil.Encode for values that it supports. Some of the tests have been
adpated from the Ethereum testing repository.

There are still TODOs in the code.

* Whisper protocol wasn't properly suppling envelope slices
* Message history wasn't properly propagated
* Added 'Messages' method, filtering any current envelope with the
  supplied filter.

JitVm struct stub. Forwards calls to DebugVm.

* proper 0xhex
* filters fixed
* start of filter manager
* accounts for ws. Closes #246