let load paths determine which adapter is required a la activerecord adapters

Attempt to let the adapter interface use the require load path instead of requiring a file to be present and static paths (that use dirname) to resolve the adapter that should be loaded.  This is the mechanism that activerecord uses and it allows specific adapter maintainers to put a "activerecord-import" folder in the load path and allow the file to be required without having to have a file present in AR-import

(AR resolver/require mechanism link below)

https://github.com/rails/rails/blob/master/activerecord/lib/active_record/connection_adapters/connection_specification.rb#L49

This may not be the "best" method, but I think it allows for flexibility in maintaining adapters while allowing the default adapters to be built in.  This idea may have already been entertained (I couldn't find an issues where it was), please let me know if I can change anything but had all the tests running locally.

(also added .gitignore entries for .ruby-* to ignore .ruby-version/.ruby-gemset)

Fixes #89
Fixes #116
Fixes #97
Fixes #90