Add getCollection and getSingle on Client

[richardlawley]

This PR introduces a number of small changes:

• Refactor code in Client.php to use getCollection or getSingle
• Modify constructor of Collection to accept a single item (so URLs like /api/tenancy/tenants/1 can be used instead of /api/tenancy/tenants/?id=1)
• Use correct URLs for collections (/api/tenancy/tenants redirects to /api/tenancy/tenants/ - works but extra request)
• Use correct URLs for single items (/api/tenancy/tenants/1 instead of /api/tenancy/tenants/?id=1)

The main purpose here was to allow users of the client to call getCollection or getSingle directly, for cases where the correct endpoint has not been defined. For example, there is no getRegions in this client. but a user can call getCollection('/api/dcim/regions/') instead now.