Refactorings and documentation changes (v0.1)

The following PR:

* Separates coded into smaller functions
* Reduces the LOC from ~190 to ~160 by DRYng header normalization
* Moves rack.lua outside of the lib folder, and into root
* Moves the example middlewares to a folder called example-middlewares
* Replaces the use of module by returning plain tables
* Adds missing info and removes inconsistencies from README
* The functionality remains the same, with the exception that rack is now loadable from any folder, instead of being fixed to the "resty.rack" path (`require` will work with other paths instead of only with `require "resty.rack"`)