Completely refactor download handling

* Added generic response content handler support
* Add to-NSString converter (response content handler)
* Add JSON parser (response content handler)
* Add image decoder (response content handler)
* Add content handlers to write to stream and file (with auto cleanup)
* Update project files and sample apps
* Update README and class documentation
Rename selective handler to selective discarder

* While it is selective to accept requests, the actual behavior is discarding
* Being stateless, it also got strapped with a `sharedDiscarder` class method
Macro cleanup

* Shorter "singleton" macro
* Correct name of header and header value definitions macro
* Improve the static string initialization a bit (when a request is created, the one-off strings)
Rename response content handler negotiation method
Make response content handlers return intermediate data on failure

* Remove 204 from acceptable response codes (I'll probably regret and roll this back someday…)
Rename convenience request creators

`getFrom:` and the likes were a bit far from naming conventions…
Block based logging

Avoids the overhead of string replacements when logging is set to silent (WARN+) levels.
Add (optional) cleanup to request content handlers
Compact error macro
Convert request context to a state machine

State transitions are now internal and, from the executor's point of view, automatic.
Update sample code
Add CURL binaries from trunk (7.28.2-DEV)

* Also added two binaries for iOS, one optimized for the app store (smaller, no support for the simulator) and other for development
* Update examples on README
Merge pull request #1 from brunodecarvalho/generic-parsing

Refactor download handling to support generic response content handlers