Enable more warnings in the project

[Eugene]

To help reduce the number of bugs in the project and catch them earlier, we should enable more warnings in the project using -Wall -Wextra compiler flags. Fixing the warnings in third-party dependencies is on a per-instance basis. Optionally, warnings should be treated as errors.

http://oleb.net/blog/2013/04/compiler-warnings-for-objective-c-developers/