Added a helper method to check if required parameters are given in an API call. Can be used
to return a `400 Bad Request` return code if a required attribute is missing.
Code clean up and fixed tests.

Information to return codes added to projects and users documentation.

Documentation is updated with information how to handle snippets or how to access tags
and commits. Nearly all project specific functions are now described in the documentation.
A few previous entries have been updated with status codes, e.g. `401 Unauthorized`.

The API documentation for projects now is structured into major sections that describe
the different aspects when dealing with projects, e.g. hooks, branches, team members etc.
All described methods now contain a list of possible status codes the method can return. A few
methods have extra sample JSON responses and a description if a method is idempotent.
Idempotent methods can be called multiple times while returning the same status code.

correctly describes the new behavior

The API accepts project names in all places where project IDs are expected. Updated the docs to
reflect that.

the documentation mentions access_level as required parameter, but fails to list
accepted values.

Added documentation for getting project hook and editing project hook.

[ci-skip]

Merge branch 'master' into non-interactive-aws-install

Conflicts:
	doc/installation.md

Fix merge mess in installation.md