Clean up gemspec

This commit makes two small cleanup changes to the gemspec file:

  - Remove commented out code
  - Convert double quotes to single quotes

These changes make the code a little more readable and a little nicer to
work with.
Add authors and summary to gemspec

These values are required for valid gemspec files.

More information:

  http://guides.rubygems.org/specification-reference/
Add middleman-syntax and redcarpet dependencies

Without these dependencies, the code will not execute properly.
Specifying them in the gemspec ensures that these dependencies will be
available at runtime.
Merge pull request #2 from lencioni/fix-gemspec

Fix gemspec