Simple Serialization Module

[hwwhww]

What's the problem?

• It seems SSZ (Simple Serialization) is likely to be the serialization protocol for Eth 2.0, so let's make a space for it.

What I did?

• Use install_requires for beacon_chain and ssz packages
• Move beacon_chain/utils/simpleserialize.py to ssz/ssz.py

TODO (other PRs): separating the tests

[djrtwo]

Looks good! Need to update readme install instructions with pip install -e .[dev]