Keep original stack of exceptions and format location as regular stack
`parse()` doesn't accept objects with a `source` property anymore.
Fixed typo in HISTORY.md
engine.parse and template.parse now return Promises
This is a preparation to allow asynchronous features like
templates/extends that might require I/O to load/validate
dependencies.
Install chai-as-promised for better testing of promise APIs
Use globals of Liquid, chai and expect in tests
Migrate liquid test-suite to chai-as-promised
Upgrade test-helper to use chai-as-promised