the original hello-world example makes little practical sense -- it is unclear what really changed when moving the slider, especially when obs is large (we always see a "bell-shaped" histogram)

let's make the number of bins reactive instead; now it is very clear what the slider really controls

a histogram with different number of bins also serves as a good demo of the property of histograms (small bins --> small variance + large bias)
add the missing Readme's and DESCRIPTION's for the examples
closes #31: try to avoid scientific notation of numbers
closes #119: we do not have to do this special treatment to the label for sliderInput, and no other inputs do as.character() for their labels

if users pass an HTML() object to the label argument, the HTML will be preserved instead of being escaped