add qhist2d()
roxygenize
changes from rstudio
closes #194: zooming is possible with mouse selection - press S and then Z

also closes #192
some of my own experiments with the layered grammar on hist2d()
re-roxygenize
add a Makefile to include some common tasks such as roxygenization
make docs
remove the cranvas_debug() function; it is rarely used
remove the pollen data; it is not wise to introduce an additional copy in cranvas from the animation package since it is already in Suggests
reformat DESCRIPTION
add the Date field and the cranvas website; remove methods from Depends because it has been imported; remove R >= 2.10 from Depends because qtbase depends on that and cranvas imports qtbase
cosmetic changes
move tourr from Imports to Suggests to speed up loading of cranvas
lazyload datasets, so we no longer need to use the data() function to load data
remove the unnecessary data() calls
we no longer need roxygenize.R; just run `make docs` in the command line
remove the man directory before roxygenizing
fixes #195: when no points are visible, the limits are calculated from the original data
bump version