Add tests for compliance with lambda spec

[brandonchinn178]

See #49

Test changes:

1. Add test for compliance with upstream lambda spec
2. Refactors integration test suite in order to implement (1)
3. Replace deprecated decode function used in test suite

Library changes:

1. Deprecate overText, instead just making Text -> Text an instance of ToMustache
2. Added docs to Text.Mustache pointing to the Text -> Text instance as the primary way to implement lambdas in mustache. This should be sufficient for most use cases.