Fixed warnings

Fixes #270 by enabling compiler warning checks + fixed all the warnings that I could find:
- conversion warnings
- Block implicitly retains 'self'; explicitly mention 'self' to indicate this is intended behavior
- Method parameter of type 'NSError *__autoreleasing *' with no explicit ownership
- deprecated methods:
  - `sizeWithFont:` deprecated in iOS7, suggests usage of `sizeWithAttributes:`, but this is only available in iOS7. Created a method `UV_sizeWithFont:` that decides which method is available and uses it
  - `sizeWithFont:constrainedToSize:lineBreakMode:` deprecated in iOS7, suggests usage of `boundingRectWithSize:options:attributes:context:`, but this is only available in iOS7. Created a method `UV_sizeWithFont:constrainedToSize:lineBreakMode:` that decides which method is available and uses it
- deprecated constants (`UITextAttributeTextColor`, `UITextAttributeTextShadowColor`, `UITextAttributeFont`)
- incompatible pointer types sending