Fence some Redis version specific functionality

redis-2.8.8 has diverged from redis-unstable in a few places,
so we need to conditionally compile based on features available
on the underlying redis version.

Note: Only Dynamic Redis >= 0.5.1 provides the DYN_REDIS_VER define
used for the functionality checking.  Dynamic Redis Unstable does
not define DYN_REDIS_VER, so absence of the define assumes *latest*
features, not *oldest* features.
Update README words for new layout details

Also fix some wording in places
Add geo commands

The geohash*.{c,h} files included in this commit are from
the e34cd4028f009f commit in mattsta/geohash-int