Add buffering for all output

Update WebServer::write(uint8_t) to write to a buffer instead of sending
single characters (or short strings) as small packets.  Remove the other
signatures of write so that the base ones in the Print class (which call
write(uint8_t)) are used instead and simplify the writeP/printP
definitions to do this too.

This greatly improves the performance of pages which are built from a
large number of small strings or values at the expense of a small
additional amount of memory (which can be adjusted with the use of a new
macro called WEBDUINO_OUTPUT_BUFFER_SIZE) and also saves ~85 bytes of
flash.