Fixes for Beta 6

These are particularly unpleasant due to the no local function can call a local function nonsense.
Don’t crash if the request URL is nil
NSTask+Promise

Tag 0.9.16
Allocate subclasses of PMKPromise
Missing #imports

Tag 0.9.16.1
Prevent compiler errors when using PMKManifold

Technically this happens whenever your block returns two different types, in those cases manually specify a return type of `id`.

Fixes #76

Tag 0.9.16.2
Confirm throwing in +new: is caught

Refs #78
ARC block/copy issues

This is worrying, but this copy fixes the crash.

I think we probably need to do more copies.

Fixes #77.
An attempt to make the code more readable
Better README examples
SKProductsRequest -> SKRequest

This is how it should have been to start with.
Unwanted Beta 7 fixes

I *know* request.URL can be nil, because I already had a crash here because of it.

I can only hope that when they say they audited for optional compliance they also checked the objective C backing code to ensure nil wasn’t possible there too.
Deprecated subspec warnings
Try to be severe in working around ARC issues

Refs #77
Refs #80
Refs #33

Tag 0.9.16.5
Fix value when manifold was used
NSFileManager+PMKPromise
Promise.when for Array<Promise<Void>>
Merge pull request #82 from hariseldon78/master

Promise.when for Array<Promise<Void>>
Fixed variable name in Swift example in README
add missing function invocation in code sample