WIP API endpoint for tags
Work on agents in tags api
Renamed api.tag to api.tags

A plural form is more consistent with the other modules in the same
package.
Post endpoint for agents-in-tag api
Removed debug log statements
Rename AgentsInTagAPI to AgentsInTagIndexAPI
Docstring updates
Use plural for tag in api endpoint for consistency
Include hrefs pointing to referenced agents

Apparently, this is more in line with the general idea behind RESTful
web services.
Full agent/job info in separate json item

When including the full info for referenced agent or job items in the
tag listing, put it into sub-object "data" instead of substituting the
object with only id and href for it.
Docstring updates
Implemented SingleTagAPI

With get() and delete() methods
No error on deleting non-existing tag

HTTP DELETE is supposed to be idempotent, so trying to delete a
non-existing tag must have the same effect as trying to delete and
existing one.
Correct returntype from json_from_request on error

What json_from_request() actually returns on error is a tuple, not a
Response object
Implement PUT in SingleTapAPI
Style: Double newlines between classes
Docstring updates