make more plotting functions available to user

[drbenvincent]

At the moment we have some plot functions available for the user to call, but we should have more (if not all) of these available. We need to be able to 'fit' a model, and then construct the figures and then call whichever plot methods we need to construct figures for a paper.

• [x] Make functions available.
• [x] Provide function help text to help the user know how to call the functions
• [x] Add tests to ensure the plotting API's don't break. Exercise some caution as we'll assume these will remain available.
• [ ] Document the plotting functions in the wiki.

This issue will go some way towards #141.

[drbenvincent]

Current plot methods available:

• plot
• plotDiscountFunction
• plotDiscountFunctionGrid
• plotDiscountFunctionsOverlaid
• plotExperimentOverviewFigure
• plotPosteriorAUC
• plotPosteriorClusterPlot
• plotPosteriorCornerPlot
• plotPosteriorDiscountFunctionParams
• plotPsychometricFunction
• plotSubjectiveTimeFunction
• plotUnivarateSummary

[drbenvincent]

At the moment we have either zero required arguments to these plot functions, or just a specification of which experiment number we wish to plot. All plot methods can take in optional varargin arguments which allows future extension to provide additional arguments to customise the plots. This will allow #141 to be dealt with.

I will avoid documenting these functions at this point... it falls into the category of 'reference' which is perhaps best dealt with by in-code help text etc. Instead, I will (at some point) add some more high-level examples in the wiki to demonstrate their use and refer back to the in-code function help for detailed info.