Cleanup of gemspec

Convert double quoted string to single quoted.
Convert arrays of string to %w-form.
Cleanup

Remove an superfluous semicolon.
Feature single string column specification

Use colon separated family:qualifier for columns.
Use array of columns for specifying what to get and delete.
Use hash of columns to values for specifying what to read.
Bump version to 0.2.2
Sync README with the colon specification changes.
Add reference to ruby-stargate in README.