Refactoring (2.1.0)

This PR introduces lots of nice features and improvements:

+ Ruby 1.8 dropped, Ruby >= 2.0 supported
+ specifications added
+ added "preprocessing chains": now you can name your file `script.coffee.erb` and first it will be preprocessed by ERB and then by CoffeeScript
+ new CLI interface: Thor instead of `optparse`, much easier for writing plugins
+ drop patching, it took too much time and it should be constantly maintained
+ introduce Gemfile support: now you can list your preprocessors in Gemfile and they will be used when using Sword; it dramatically boosts boot time since you manually declare dependencies so now we don't load anything unnecessary
+ drop "20 lines of core", now it is 60 readable and easy to understand lines of Rack middleware
+ `sword compile` task is added, tested and ready to rock
+ much, much faster; server has absolutely new and much better logic

The release is bumped to 2.1.0.

Some work to be done to officially release the gem:

+ finish `sword init` task: it should create a Gemfile with comments and maybe a index.erb page or whatever
+ release [sword-branch](https://github.com/yegortimoschenko/sword-branch)
+ release [sword-deploy](https://github.com/yegortimoschenko/sword-deploy)
+ release [sword-info](https://github.com/yegortimoschenko/sword-info)
+ release [sword-compass](https://github.com/yegortimoschenko/sword-compass), see code from deleted [compass.rb](https://github.com/yegortimoschenko/sword/blob/a50a22f0d4b88e8d8476877706bfc5aba3d9a0ea/lib/sword/patches/compass.rb)
+ should think about some asset pipeline support, see #21; maybe use Sprockets, maybe write something compatible as a plugin
+ write good documentation and make a good website

This is absolutely stable, and anything in `master` will be stable. Testers are welcome.

/cc @artemeff @CodingFu @rozzy