Addresses#952This adds a method to the `ProjectManager` called `import_github`, whichmaps to the `/import/github` API endpoint. Calling `import_github` willtrigger an import operation from <repo_id> into <target_namespace>,using <personal_access_token> to authenticate against github. Inpractice a gitlab server may take many 10's of seconds to respond tothis API call, so we also take the liberty of increasing the defaulttimeout (only for this method invocation).Unfortunately since `import` is a protected keyword in python, I was unableto follow the endpoint structure with the manager namespace. I'm open tosuggestions on a more sensible interface.I'm successfully using this addition to batch-import hundreds of githubrepositories into gitlab.

On startup, the `timeout` parameter is loaded from config and stored onthe base gitlab object instance. This instance parameter is used as thetimeout for all API requests (it's passed into the `session` object whenmaking HTTP calls).This change allows any API method to specify a `timeout` argument to`**kwargs` that will override the global timeout value. This wassomewhat needed / helpful for the `import_github` method.I have also updated the docs accordingly.