* inheritance of directives: using meta-classes to inject attributes coming from directives into packages + lazy directives
* _dep_types -> dependency_types
* using a meta-class to inject directives into packages
* directives are lazy
fixes#2466
* directives.py: allows for multiple inheritance. Added blank lines as suggested by @tgamblin
* directives.py: added a test for simple inheritance of directives
* Minor improvement requested by @tgamblin
CMakePackage: importing names from spack.directives
directives: wrap __new__ to respect pep8
* Refactoring requested by @tgamblin
directives: removed global variables in favor of class variables. Simplified the interface for directives (they return a callable on a package or a list of them).
yaml-cpp has a boost dependency, and according to [yaml-cpp
page](https://github.com/jbeder/yaml-cpp):
yaml-cpp 0.5.3 has been released! This is a bug fix release. It also
will be the last release that uses Boost; futures releases will require
C++11 instead.
