Add LevelManager class
Add level builder
Fix tests
Add high level level configurer
Minor review fixes
Move level-related classes to ./level directory
New format of makefile

This allows `bem build` and `bem create` commands to use level manager
Make is chainable
Fix review issues
Replace setConfig with setBundleBuildLevels
setDefaultTechs is consitent with the rest of API
Correctly resolve techs from $ROOT/.bem/techs
Resolve npm modules from root of a project
All sections in make is optional
Level manager works in bem server
All level builder varargs methods accepts arrays
Fixed code review by @diunko
Remove redundant check
Merge pull request #468 from bem/feature/level-manager

LevelManager+LevelBuilder.