ENH: Extending TrialHandler's n-back information options

In response to user query at
<https://groups.google.com/forum/#!topic/psychopy-users/9aeU1AgEJv4>.
Sometimes access is needed to information from previous trials (as in
an n-back task). As a kludge, the trialHandler's getFutureTrial()
function could be used for this by passing a negative index value, but
as it wasn't designed for this, it didn't include a bounds check to
handle trying to access a trial before the first one. This has now been
added.
To be explicit, have also added a getEarlierTrial() function, which
simply calls the getFutureTrial function but ensures the index is
negative.
Merge pull request #338 from m-macaskill/master

	ENH: Extending TrialHandler's n-back information options
DOC: Fix for namespace issues in psychopy.data API examples

A user found problems when trying to create a Multistairhandler:
<https://groups.google.com/forum/#!topic/psychopy-users/kPEtdAp5C_s>

This was due to the docs not specifying that handler classes are
instantiated with a "data" namespace prefix. Fixed this to make all of
the examples in the comments consistent (previously the correct syntax
was given only for one QuestHandler example).
Merge pull request #342 from m-macaskill/master

DOC: Fix for namespace issues in psychopy.data API examples
DOCS: updated workshop details
Merge branch 'master' of github.com:psychopy/psychopy