Add "path_prefix" Connection option to allow for simpler consumption of RESTlike APIs

The "base_path" option is used to set a base path for a web service end point.  Calls to `request`, on a `Connection` with base_path set, merge `base_path` and `path` into a single path that is then passed to `excon`.

Tests were added to ensure that the default behavior, in the absence of `base_path`, continue to operate as before.

### Rationale

We've found, while working on `fog-openstack-core`, that we very much want a feature like this on Connection.  Otherwise, we are either hardcoding full paths in our individual request methods or delegating responsibility for munging the path somewhere higher up in the class family.  Both of these seemed suboptimal.