order var declarations in schema
Add support for loading var from file
Add initial implementations of components for extensible variable loader
Test top-level var loader
Add schema checking for vars
Move environment context to utils module
Better check for variable type
Add helper to load vars from other parts of test (e.g. templated URL)
Improve some var-related exceptions
Use new var loader in config and web test
Update example config
Merge pull request #10 from sjagoe/file-var

Restructure variable loading for extensibility