Add tests

[LukasKalbertodt]

This crate should have tons of automated tests. Now that the API is somewhat stable, I can start adding them. An incomplete list of things that need testing:

• template: false with template syntax in it
• serve: false
• dynamic: true
• Assets in subdirectories of the base path
• Cyclic include with 2 files
• Missing variables
• path overrides
• Different runtime base path