Support only running tests annotated with a TestSize. Fixes #124

* Adds a --size command line parameter that takes 'small', 'medium',
  or 'large' as a value
* Adds a small bit of text to the HTML output indicated which size
  was run
Fix typo in README
Changes based on @JakeWharton's comments

* Formatting changes
* CLI parameter description
* Move TestSize formatting into presentation layer
Merge pull request #130 from AutoScout24/scompt/test_size

Support only running tests annotated with a TestSize. Fixes #124