source reference qualifiers

We've identified the need to provide additional "qualifiers" on a source reference. Examples of source reference qualifiers include:

* Providing a page and paragraph on a citation to a document.
* Identifying a specific region (i.e. "bounding box") on a citation to a digital image.
* Identifying a specific segment on a citation to an audio or video recording.

Your comments are invited on the attached proposed changes. The changes can be summarized as adding a "qualifiers" property to the source reference object. A qualifier is basically just a name-value pair, the name being an element of a constrained vocabulary. The following source reference qualifiers are also specified:

name | value
-----|-------
`http://gedcomx.org/Page`|A page in a document.
`http://gedcomx.org/Paragraph`|A paragraph in a document.
`http://gedcomx.org/CharacterRegion`|A region of text in a digital document, in the form of `a,b` where `a` is the start character and `b` is the end character.
`http://gedcomx.org/RectangleRegion`|A rectangular region of a digital image, in the form of `x,y,w,h` where `x` is the point on the X axis of the image in pixels, `y` is the point on the Y axis in pixels, `w` is the width of the rectangle in pixels, and `h` in the height of the rectangle in pixels.
`http://gedcomx.org/PolygonRegion`|A polygon region of a digital image, in the form of `x,y,x,y...` where `x` is a point on the X axis of the image in pixels and `y` is a point on the Y axis in pixels.
`http://gedcomx.org/TimeRegion`|A region of time of an audio or video recording, in the form of `a,b` where `a` is the starting point in milliseconds and `b` is the ending point in milliseconds.