Exclude problematic gems on Windows

* Newer versions of `bluecloth` [do not compile on Windows][1] out of the box
* `handlebars` depends directly on `therubyracer` which in turn depends on `libv8` which [does not compile on Windows][2] out of the box either

[1]: http://stackoverflow.com/questions/4932221/bluecloth-v2-0-10-with-windows-7-not-working
[2]: http://stackoverflow.com/questions/10812696/problems-with-jslint-v8-ruby-gem-installation-on-windows7-64-bit/10924081#10924081
Use `where` to check for external commands on Windows
Skip "succeeding" rsync tests when rsync is not available
Skip tests involving symlinks when symlinks are not available
Use Bundler::WINDOWS to check for Windows
Skip pygments.rb tests on Windows

`pygments.rb` does not support Windows in it's current version since it uses
`posix-spawn` which assumes a POSIX environment.
Amend regex to allow it to match with CRLF too
Revert Gemfile.lock
Amend skip message for missing symlinks
Introduce `on_windows?` helper
Simplify skip_unless_have_command
Split command and symlink checkers in a "have" and a "skip" method each
Fix some rendering tests breaking following test
Improve missing command message
Skip less for installations on Windows since it depends on libv8
Fix test_parse_utf8_bom on Windows
Fix unwanted output when testing for available command
Merge pull request #353 from raphinesse/fix-tests-on-windows

Fix tests on Windows