Export types used by decode/encode

[webmaster128]

Closes #58

[webmaster128]

Hmm, sorry about huge diff. The basic idea was to move all the code into lib.ts and use index.ts for explicit package exports.

[coveralls]

Coverage remained the same at 90.991% when pulling fecee84 on webmaster128:rlp-type into ee8a7f5 on ethereumjs:master.

[holgerd77]

Ah, please not such large side reorgs on such a tiny change, won't approve this

[webmaster128]

Yeah, updated

[holgerd77]

Thanks, cool! This generally looks a lot cleaner than our previous RLP*-prefixed version.