Initial erasure-coding of availability data (#56)

* erasure-coding block data

* adjust error handling

* merkleize chunks and yield branches for each

* construction and proving of merkle branches

* port over to new GF(2^16) impl

* some tests for wrapped_shard

* handle extra byte from GF(2^16) better

* point to github dependency

* add issue link

* point to master for reed-solomon-erasure

* add missing license header