Modernisation

Major changes:

* Use multiple files of a reasonable size
* Use roxygen2 for documentation and namespace  (note that you'll need to development version since I had to fix a few S4 related bugs)

I did a reasonable amount of cleanup on the docs, particularly removing what I think were incorrect aliases (e.g. `dbWriteTable-methods`, and generally splitting out the docs into more smaller files. I also documented the sqliteXXX functions with the methods. These should probably all be internal, but that's a big change I didn't want to make here.

I think this is ready to be merged back in - it passes R CMD check (including all tests).  I can't guarantee that I didn't accidentally change any code (and it's a massive diff so hard to review by hand), but I tried my hardest not to.