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