Add a tox job for generating docs

Add a tox job to make it easier for developers to generate their
own docs before submitting a patch.

Change-Id: I86dbfc64a9d174697048c254c6d3d042a62350b4
Add importable SwiftService incorporating shell.py logic

This patch adds a SwiftService class that incorporates the high
level logic from swiftclient/shell.py. It also ports shell.py to
use the new class, and updates the code in swiftclient/multithreading.py
to allow the SwiftService to be used for multiple operations whilst
using only one thread pool.

Change-Id: I08c5796b4c01001d79fd571651c3017c16462ffd
Implements: blueprint bin-swift-logic-as-importable-library