Separated touch and delete

Apparently, the delete command does not really support a time parameter
even though the code had support for it -
https://github.com/memcached/memcached/blob/master/doc/protocol.txt#L254
Also, found that the expiry parameter is mandatory in touch -
https://github.com/memcached/memcached/blob/master/doc/protocol.txt#L324.
For these reasons, it makes more sense to use separate functions.

(Note - easier to review this change comparing to the version before my previous pull request - 
https://github.com/erankor/python-memcached/compare/794b02811d8c6fdad304955a4602fb1324270f95...e0ba0fc1fd30a3c1f1a06aea526d1e53f1850dfe)