When building code relying on this library in Xcode, the build process throws an error (not a warning!) stating its inability to locate the presumed system includes. The `<>` angle brackets on `#include` indicate a system-level include, not a user-level include. By swapping to quotation marks for the `#include` we more accurately represent the content of the included files and Xcode builds without complaint!
- updated documentation wrt. new repository layout
- temporarily switched off Homebrew --HEAD building (can only be switched on after release)
- set copyright date to 2018
* Rename 'develop' folder to 'include/nlohmann'
* Rename 'src' folder to 'single_include/nlohmann'
* Use <nlohmann/*> headers in sources and tests
* Change amalgamate config file
