in XML deserializations, we can get get floats for start/stop times, so we need to be able to parse those

add test video with float-based end-time

change from_xml class method on video class to make sure we honor the fact that the xml string could actually be a pointer to another file on disk which actually contains the attributes

add a default XML serialization if the 'data' field is empty or None. Some XModules now - like video XModule - do not have XML set in the 'data' field

Also fixed things in the toy course where needed

Factories were used when possible but tests depending on importing or things not manageable by factories used the toy course.  New files were added to include the needed functionality.  Most files are blank or have minimal xml needed to work