README: render markdown on Github web gui

The content is already markdown format, so rename README{,.md}.
Github only renders markdown if the file has a recognized extension [1].

[1] http://git.io/j2DmoQ
spec_helper: improve exception handler

* print to stderr, not stdout
* rescue from StandardError, not Exception [1]

[1] http://goo.gl/r3nFG
spec_helper: avoid traceback with minitest 5

    $ bundle exec rake spec
    /usr/bin/ruby -S rspec spec/unit/chocolatey_spec.rb --color
    Warning: you should require 'minitest/autorun' instead.
    Warning: or add 'gem "minitest"' before 'require "minitest/autorun"'
spec_helper: avoid NameError exception on Minitest 5

Addresses:

    MiniTest::Unit::TestCase is now Minitest::Test. From
    /usr/share/ruby/test/unit/testcase.rb:8:in `<module:Unit>'
    ======================================================================
    NameError
    uninitialized class variable @@installed_at_exit in Test::Unit::Runner
    ======================================================================
Gemfile: require Minitest 5

Before this commit:

* fresh bundle gets Minitest 5
* existing bundle still has Minitest 4.x and
  may fail due to minitest/autorun in spec_helper

After this commit:

* all bundles get Minitest 5
Gemfile: Add travis and travis-lint

* Add travis gem to provide command-line interface, such as
  the `travis sync` command

* Add travis-lint gem to check validity of `.travis.yml` config
spec: add unit tests to validate .travis.yml

New unit tests:

    .travis.yml
      exists
      is a valid travis-ci configuration
travis: add current puppet versions to test matrix
travis: remove old puppet versions from test matrix
travis: add ruby 2.0 to test matrix

Before this commit: Travis runs 4 tests on each pull request...

    Rvm   Env
    1.8.7 PUPPET_VERSION=2.7.22
    1.8.7 PUPPET_VERSION=3.2.2
    1.9.3 PUPPET_VERSION=2.7.22
    1.9.3 PUPPET_VERSION=3.2.2

After this commit: Travis runs 5 tests on each pull request...

    Rvm   Env
    1.8.7 PUPPET_VERSION=2.7.22
    1.8.7 PUPPET_VERSION=3.2.2
    1.9.3 PUPPET_VERSION=2.7.22
    1.9.3 PUPPET_VERSION=3.2.2
    2.0.0 PUPPET_VERSION=3.2.2
git: show smarter diff for ruby files
git: ensure ruby files have LF line endings
README: improve markdown formatting

* Use syntax highlighting for puppet example
* Use literal tags for inline samples
* Add links to chocolatey and puppet web sites
README: show how to set chocolatey as default provider
CONTRIBUTING: add instructions to contribute

This doc is shown to people who open a pull request on Github
as described at https://github.com/blog/1184-contributing-guidelines
README: Use correct upstream build status for travis badge

@rismoney transferred the upstream repo to the chocolatey org
a few months ago. It makes sense that the build status should
be badged against the upstream org.
README: Convert README with dos2unix line endings
travis: do not run `bundle install` twice

Before this commit: travis runs `bundle install` twice

After this commit: Rely on travis default behavior to
run `bundle install` automatically before testing.

http://about.travis-ci.org/docs/user/languages/ruby/#Default-Test-Script
README: kernel fact is windows, not Windows
README: add installation section

provide links to common installation techniques