SQL escaping/quoting proposal

I've cleaned up the examples and made them work and put the results in
https://gist.github.com/hadley/7057387 - that should make discussion a
bit more concrete.

I made a few more changes to https://gist.github.com/hadley/7057387

* the method is now based on the connection, not the driver, since for
some drivers the escaping may vary between connections (e.g. ODBC)

* I added a sample tableName function (which I don't think would be
included in DBI), showing how you might accept multiple forms of table
names, and do the right thing as much as possible.

Hadley

Chief Scientist, RStudio
http://had.co.nz/