v1.5

This 1.5 version brings a **lot** of improvements:

- Real documentation: a docset is now generated and hosted on this repo, and will be hosted on [cocoadocs](http://cocoadocs.org) too.
- `speedRate` and `remainingTime` properties have been added to `TCBlobDownloader` objects thanks to @albertschulz!
- The code has been refactored and improved for a better maintainability, same for the test suite
- Some more tests

Visit [1.5 branch](https://github.com/thibaultCha/TCBlobDownload/tree/v1.5) to see the updated repo while this PR is waiting to be merged.

A few more things to go:
- [ ] Fix the delegate methods unit tests (testing if those methods are being called on main thread as expected). This might wait for a `1.5.1` version.
- [x] Investigate a weird behavior of the remaining time property during the first two or three seconds of a download. It might be due to the fact that we are lacking of speed samples during the first seconds of a download, and with more samples, the remaining time property is stabilized.

Any code review or suggestion is welcomed!