Add phpdocs for Redis commands in Predis\ClientInterface.

This commit addresses a long-standing feature request posted in #89.
Added Predis\ClientContextInterface.

This interface replaces Predis\ExecutableContextInterface and it is
used by client-side contexts such as pipelines or transactions.
Added @method tags to phpdoc header of Predis\ClientContextInterface.
Miscellaneous fixes for "use" directives.
Merge branch 'v1.0/phpdocs'