Add ChainSpec structure.

It includes all constants from the
[Constants](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#attestation)
section in the spec, except for things that are clearly enums.

My reasoning is that these enums are not so much "chain specification"
and this struct should be reserved for items that "configure" the chain.

beacon_chain/spec/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "spec"
+version = "0.1.0"
+authors = ["Paul Hauner <paul@paulhauner.com>"]
+
+[dependencies]
+types = { path = "../types" }