Group API calls into sub-classes

[aik099]

The Jira API is huge (lots of API calls). With time we could end up having all of them present in the Api class, which would produce a huge class.

Proposing to create the separate classes based on API call groups in Jira REST API itself and move corresponding calls there.

For example:

// Before:
$api->addWorklog(...);

// After:
// $api->worklogs->addWorklog(...)

For BC reasons a __call method could be added, that would forward calls to unknown methods to the corresponding classes.

The properties themselves would be proxy classes (a lightweight proxy class is always present, but a heavy underlying class object is created only when needed).