Added commands for working with "sets"

- setadd <key> <value>
- setremove <key> <value>
- setmembers <key>
- setismember <key> <value>
[addtoset] Fixed typo
[sets] Changed naming of set commands

From set(add|remove|members|ismember) to s(add|del|members|ismember) to
avoid confusion with the normal `set` command.
[sets] Minor adjustments
[sets] Properly parse arguments to the sadd command
[sets] Fixed naming mistake
Merge pull request #5 from emilisto/command-addtoset

Added commands for working with "sets"
[sets] Updated README examples