Revision 197801

1.) Driver constructor modified to require RenderType, and setter functions
for RenderType and FOUserAgent removed, to allow for better checking of
a valid Driver object at time of construction.  (Also, will reduce need to
validate Driver object should it be a parameter to a method.)

2.) Added validity checking of renderType to FOTreeBuilder as well, in
effect decoupling Driver from FOTreeBuilder (the latter can work on its own
now.)

3.) Updated the various examples and the documentation to indicate the
new API.