Mostly cosmetic changes

This is mostly cosmetic.  The changes include:

* Using a single routine to construct the `<mrow>` that replaces `<mfenced>`, since essentially the same code was used in three or four places.  I also added the `texClass` property that the original mfenced implementation used.

* Fixed up some uses of `var` to avoid using it twice for the same variable within a function.  Technically, that is an error, though the browsers don't complain.  The packer will, however, so it is helpful to avoid it.

* Fix up some spacing and comments.

* Replaced use of `.trim()`, since it is not supported in early vesions of IE.

* Used `firstChild` rather than `firstElementChild`, etc., as these are not supported in IE.  This is OK in this case because the MathML created from the internal format doesn't include any text or comment nodes where there should be element nodes.

* Used `Get()` to get the `fenced` property, so that it will be looked up from the dictionary if it isn't given explicitly (so it will recognize the construction in more cases).

* Used `data.join("")` rather than `data[0].data[0]`, since a token element technically can have more than one child text node.