Properly document and maintain contract behavior test files

[nventuro]

It'd be nice if we could provide our behavior tests (e.g. Ownable) to users, so that they could in turn check that contracts inhering from those still preserve the base behavior (if applicable) and that it hasn't been accidentally broken (e.g. by overriding a modifier, etc.).

Related to #326.

[frangio]

They should also be properly documented, including what "arguments" they expect present in this.

[frangio]

Note that they are already exported.

https://github.com/OpenZeppelin/openzeppelin-solidity/blob/66c19689137b522555cab7d8a6a158c18fa443b4/package.json#L5-L8

I think this issue is more about documenting them and making sure they are usable.

[frangio]

Additionally, we might want to establish a rule as to what behaviors we will provide (if not for all contracts, which subset?), so that users know what to expect.