v0.7: API change

BREAKING API CHANGE

call() and call_async() now return a Result<String> instead of plain String to avoid losing the error.

v0.6: rewritten core, now with mocking.

I move to String internally instead of &str. Easier to deal with and use mocking for tests. API should stay the same.

Version v0.5.0, first stable API.

Welcome v0.5.0: only reqwest now supported.