Initial version

I moved out all files and did some fixes in library (see the email).

What needs to be done now:
* merge this PR
* add tests to be executed on Travis CI
* fix `RebillyCustomerTest::testThrowsExceptionOnNullOrNotsetEnv` its test specification is not correct
* add to packagist
* write a readme (with installation intructions, and test execution instructions)

I cleaned up tests a bit. Added Codeception\Verify for some tests readbility. Prepared project for packagist - created composer.json with the right autoloading settings.