API: Implement edit via API for projects

Implements edit via API for projects. Edit was part of missing features in feature request Full CRUD operations via API for projects. 
http://feedback.gitlab.com/forums/176466-general/suggestions/3904506-full-crud-operations-via-api-for-projects

Feature is implemented using existing UpdateService for projects. Permission to change visibility level and name are checked in addition to check for permission to administer project.

Doesn't allow updating project namespace id, because there was existing API-method for transferring project to a group.

Documentation added to doc/api/projects.md. Uses API request *PUT /projects/:id* .

Tests included for:
  1) Success for changing path
  2) Success for changing name
  3) Success for changing visibility level
  4) Success for changing all other attributes
  5) Success for changing name & path to existing name & path but in different namespace
  6) Failure if not authenticated
  7) Failure if path exists in project's namespace
  8) Failure if name exists in project's namespace
  9) Failure if not sufficient permission to change name
  10) Failure if not sufficient permission to change visibility level
  11) Failure if not sufficient permission to change other attributes

Allows updating following parameters:

- name
- path
- visibility_level
- public
- default_branch
- issues_enabled
- wiki_enabled
- snippets_enabled
- merge_requests_enabled
- description