Added first implementation of context stack
Removed 'global_context' parameters (now implemented by ContextStack object, refactoring in renderer and models modules
Muspyche api module reflects the changes done to backend
Better code for spec tests
Added some unittests for context stack implementation
Context will now look for keys on stack and in current context
Fixed rendering of partials
Changed dot_access atom test
Modified template and expected text for section atom test