Re-organizing and cleaning up README

I found the current README a bit confusing and decided that I could put some effort into reorganizing it. Below a summary of what I've done so far.

* Made all url's https where available
* Added more consistency with code examples, capital usage, paths
* Created a structure by using headings
* INSTALL was confusing, tried to make more sense
  * Added "Composer" section
  * Re-arranged installation methods
  * Removed unnecessary steps
* Removed double spaces where appropriate
* A few other minor edits / corrections