Remove TIFFs from specs

On Mac you have to compile ImageMagick specially with ability to process
TIFFs, and it isn't worth it, because we don't test anything with it.
Comment unclear code
Make MiniMagick::Image a bit DRYer
Remove MiniMagick::Image#<<

This method is too unexplainatory, it isn't clear to an outside reader
what it does. We can just use this method in `#combine_options`.
Reveal intention
Refactor image attributes

We want to encourage nice attribute readers rather than the superscript
operator. Superscript operators doesn't care about typos, while Ruby
methods do.
Fix RDoc issues
Add Image#resolution

Fixes #164
Completely avoid escaping
Check if #processor has valid value
Make dynamic operator methods introspectible