Refactor initial import and syncing

This will refactor the `get project` command and remove the project specific sub commands `comments`, `events`, `issues`, `labels` and `milestones`.
The command is now meant to be run on a periodic base, looking for modified issue descriptions and comments, while making as few requests as possible.

During the initial import of issues, when setting up the project, a large amount of requests might be necessary, depending on the amount of issues and their comments.
A mechanism has been implemented to allow more then one account to be used in those circumstances.
To achieve this, the GitHub object has been extended to check for the remaining GitHub rate limit.

New options:

* `issue` Process only a single issue.
* `range_from` First issue to process.
* `range_to` Last issue to process.
* `force` `f` Force an update even if the issue has not changed.

A new column `updated_date` as been added to the `#__activities` table, mainly to check for modified comments on GitHub