ENH: Add ExternalData Management System

Download Data from Girder according to data key files contained in iMSTK/Data

This uses ExternalData.cmake (Requires CMake >= 2.8.11 release).

Add TEST: Add ExternalDataTest to verify downloaded files from Girder
* Be sure the data requiered for the test is not there (delete it)
* Add the imstk_add_data(${target} ${data_list}) function in CMakeList.txt
which calls ExternalData_expand_arguments().
* It downloads the data provided by the data_list (relative path from
iMSTK/Data)

Add DOC: Add documentation on ExternalData Management
* How to add data to iMSTK from Grider
* Update data
* Use data