various changes, beginning of support for Internet Explorer

- only run getBBox once and save to variables text_node_height and
text_node_width
- use a variable for the string split delimiter to keep track of
whether we’re splitting along word breaks or force-wrapping a character
string with no breaks
- store total computed offset as a more readily accessible property
alongside each substring instead of rebuilding with a loop
- beginning support for internet explorer, which is to say that the
switch does work and fire in tspan mode, but the dx attribute isn’t
properly assigned yet so they substrings are not wrapping to a box so
much as zipping around the screen to various inconvenient locations