Add iostream.write_buffer_size property.
Add max_write_buffer_size to iostream.
Remove `write_buffer_size` read-only property.
Merge remote-tracking branch 'methane/write-buffer-size'

Conflicts:
	tornado/iostream.py
Document max_write_buffer_size.

StreamBufferFullError is no longer an IOError, and is now used for
the read buffer as well.