Added nullability annotations to Bolts Tasks.

[nlutsenko]

Closes #161

This pull request adds nullability annotations to Bolts Tasks.
Few highlights:

• This is somewhat potentially breaking, since there are no more implicitly unwrapped optionals in Swift after this is done
• This will work on any Xcode 6.3.1+ (which is going to be a requirement after this is merged in) (I can make it support versions before, but it will get way more polluted, and not sure on the actual impact)
• For qualifiers - this is still using __nullable instead of _Nullable, since the latter is available on Xcode 7.0+ only

Everything is split into separate commits for easier review, will squash later.

cc @grantland @hallucinogen

[arielelkin]

👍

[facebook-github-bot]

@nlutsenko updated the pull request.