Makefile fixes

Few fixes to Makefile:

* Separate out install from build so un-privileged user can build
* Install header files to /usr/local/include/RF24 for easy inclusion when using this library: i.e. ```#include <RF24.h>```
* Updated readme.md to reflect changes
* Cleaned up readme.md while I was in there
  * Use consistent syntax for headers
  * Tabs -> Spaces
  * Indent code by 4 spaces rather than using backticks
  * Fix table so it renders correctly in github