First beta of SDWebImage 3.0

This version is a rewrite of the lib using iOS 5.0 as minimum target.
The lib as thus been refactored to use blocks/GCD from the ground up.
As a consequence, all delegate code style has been removed. The result
is a lot less lines of code and more flexibility.
Add who use it section
Remove outdated motivation section
Add "How is SDWebImage better than X?" wiki link
Add Licenses section
Fix typo in license URL
Fix wrong cache policy used (fix #211)
Setup NSURLRequest

- Activate pipelineing
- Disable cookie support
- Add image/* Accept header to force image on servers handling content negociation
Fix source file headers
Update podspec
Prevent caching of responses by NSURLCache (fix #211)
Fix fucked-up merge
Fix example app
Add progress block support to UIImageView category
Fix aggregation tracking urls not cleaned in case of cancellation
Track leaks
Do not use dispatch_io_write as it generate random crashes

Either it's buggy under iOS or I didn't use it the right way...
Use global low prio queue instead of a dedicated queue for caching I/Os
Use dispatch_barrier to handle NSMutableDictionary thread unsafety instead of main thread dispatching
Ensure we're not decoding the image in the main thread