add a changelog file

The changelog is initialised with the changes being made to the bundle
since the release of version 1.0.0. It has to be kept up-to-date when
new features are implemented or when bugs are fixed.
make tests work on HHVM

Mocking the ApiException leads to a bad method call exception on HHVM.
Since mocking the exception has no real value, using an instance of
the exception itself is a sufficient fix. Now, tests run properly on
HHVM enabling it to verify full HHVM support.