Do not use shellescape on Windows + Remove Gemfile.lock

`shellescape` (called inside `shelljoin`) was designed for Bourne shell, and it mangles Windows paths (e.g. `C:\My Install Path\wkhtmltopdf`). As per [this stackoverflow answer](http://stackoverflow.com/questions/5636601/is-there-an-equivalent-to-the-ruby-shellwords-module-for-the-windows-shell), there is no equivalent of `shellescape`  on Windows platform.

As an alternative, I've made a simple join/quote-escaping scheme for Windows. I haven't given a detailed look at injection vulnerabilities on Windows. For the time being this will have to be a caveat for Windows users.

Also removed Gemfile.lock. Please see [this post from the folks who made Bundler](http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/)