Document FullStackTest such that it is more visible

[meetmangukiya]

I am...

• [x] Here about something else

Issue description

FullStackTest allows for unittest style testing, it already exists, but is missing from documentation at errbot.io. Maybe make it more visible to the users :)

https://github.com/errbotio/errbot/blob/master/errbot/backends/test.py#L466

[andrewthetechie]

@meetmangukiya could you be more specific about what you are looking for documentation on and what is missing? Your link seems to go to a blank line in backends/test.py

[meetmangukiya]

I guess there are new commits since then, changing that file. New position would be: https://github.com/errbotio/errbot/blob/master/errbot/backends/test.py#L487

FullStackTest class can be made a base class to write unittest style tests.