Change API of bem-environ to one more straightforward

- make environ exported object a function to set global root
- make lib/arch export a function that accept nodes registry and environ object as its arguments
- add environ.extendMake() function to apply nodes extensions
- reflect changes in a readme
Remove --save-dev from readme
Merge pull request #6 from bem/feature/refactor-api

Change API of bem-environ to one more straightforward