Add HMSETNX command
Update readme to use more general example branch

Instead of pinning the example to 2.8.8 the example
now uses the floating branch 2.8 that receives commits before
they get bundled into an official release.
Update commands for 2.8.9-0.5.2+ compatibility

Now we: don't use the [kinda broken idea] dynamic version define
and instead use corase feature detectors defined in Dynamic Redis 0.5.2