Added NSNotifications being posted on every network request being sent/finished.

[nlutsenko]

This is required #74

• Added delegate to PFURLSession
• Updated Tests
• Added NSNotification posting whenever the request will be sent/finished.

[richardjrossiii]

I'm not super happy about using the default NSNotificationCenter everywhere. Thoughts on having a Parse-specific dependency-injected one? It would make testing/mocking better I think.

[richardjrossiii]

If we fail below the protocol level (e.g. user switches wifi off), this will be called without a URL Response, and no way to get the error out. Thoughts on having a way to get the underlying network error if one exists here?

There's also no good way to track cancelled requests.

[nlutsenko]

For this specific case - we don't really care about the result/error/cancellation, only about when the request is about to be sent and when request succeeded.
We don't really need an error right now, or a result, or cancelled status - that's why these are not implemented.

We can expand from here though at any given point of time.

[nlutsenko]

Parse-specific - not really
Dependency-injected - sure

Will update in a bit.

[richardjrossiii]

This is bad. Self may be re-assigned by init, and then we have a dead delegate value. We should think about how to improve this.

[nlutsenko]

Sure, the possibility of that happening is really really close to 0, so better testability (with injected everything) vs rare edge case scenario.