* Add shard blocks, shard data roots and how data is computed into crosslinks
Includes:
* Shard block structure
* Shard block header verification rule
* Shard block fork choice rule
* Shard block body verification rule
* Crosslink verification rule
Possible simplification: require `calc_block_maxbytes` to always output an exact power of two; if we desire the average maxbytes to be smooth, we can simply make it a pseudorandom chose between powers. This reduces some of the padding complexity.
* create separate files for phases (#125)
* create separate files for phases
* fix links
* add shard block pre processing conditions
* cleanup
* remove 'essentially'
* Updated handling for beacon chain skipping slots.
* Handle missing slots more
* modify attestation validity rule for crosslink hash
Chih Cheng Liang
vbuterin
Ben Edgington