minor: making some readme improvements

* slimmed down existing readme, made it easily consumable (the "5 min readme")
* added clear sections for compatibility, installation, usage, etc.
* moved mixed tutorial content into the wiki and provided a link
* updated text all over, made consistent with 2.0 driver readme
minor: typo in ruby platform support for 1.x version of the driver

We don't actually support rbx in the 1.x driver. This was an accidental copy from the 2.x readme where we do support rbx.
Merge branch '1.x-stable-ruby-676' of github.com:gjmurakami-10gen/mongo-ruby-driver into gjmurakami-10gen-1.x-stable-ruby-676