[CDF-287] - As a user, I want to be able to define a chart made up of arbitrary, compatible, plots.
* Ability to define multiple plots of arbitrary types, through the new chart option "plots"
* Ability to configure existing special plots through the "plots" option given its name ("main", "plot2", "trend")
* Ability to configure each plot's visualRoles at the plot options level and of sourcing these from other plots' visual roles (by using the scheme "plotNameOrId.visualRoleName").
* Improved logging of visual role mapping information (as well as for the other log sections: data source, complex type, etc).
* Ability to specify extension points directly at the chart options level or at the plot options level, outside of the corresponding "extensionPoints" map.
* Changed the plot type of the normalized bar plot from "bar" to "nbar" (breaking change for some advanced uses).
* Reorganized the code:
  * consistent naming of files, uses a class suffix for better identification in a folder having different kinds of files
  * grouped files by visualization class, favouring the individual packaging of visualizations (in the path to AMD)
  * separated the pvc.data namespace into the "cdo" package (acronym for Community Data Objects), a sibling of "ccc" and "def".
  * split the giant pvc.js into multiple files.
  * moved some of the framework-related functionality in CCC to def.js
  * made it easier to update the "pvc.build.js" file by providing a script that converts the contents of "ccc-bundle-files.txt" into a JavaScript array "ccc-bundle-files.js", ready to copy&paste in the "pvc.build.js"

* Updated the code style
* Fixed an in the mean time revealed bug of the interpolation tests, that did not clear caches after changing the visible state of datums (the groupBy was previously done on a linked partData that now is spared of being created in some cases; there was no cache on the root level before).
* Fixed global leak in the number format tests, that left the global groupSizes changed, and affected other unrelated tests.
* Fixed an issue in the visual roles binding code that would prevent defaulting dimensions' properties from the properties of the single visual roles bound to them.
* Ability to not specify the "plot2" option, being sufficient to specify a spec for the plot named "plot2" in the "plots" option.
* New option legendVisible of visual roles. Allows hiding the legend group of only a particular plot and axis.
* Unit tests
* Options JsDocs
Merge pull request #97 from dcleao/CDF-287

[CDF-287] - As a user, I want to be able to define a chart made up of arbitrary, compatible, plots.