Cleanup README.md and LICENSE

* Drop the OpenBSD related found in the LICENSE file as we refer
  to BSD 2-Clause License.
* Refer to contributors via the automatically generated list.
* Clarify headers and drop statement that we are running on
  Travis CI which can be easily seen in the build status header.
Contributors as link which is much more readable.
Get rid of obsolete information in README.md

* Drop statement referring to initial Ruby and Ruby on Rails version
  which doesn't provide any helpful information anymore.
* Drop statement encouraging user to seek for help in Rails Guides
  which doesn't make sense anymore as we already tagged version 1.0.
Merge pull request #74 from h5b/master

Cleanup README.md and LICENSE file