add smoothing to plothist2d

This adds option to smooth 2 dimensional histograms with gaussian kernel (or by user defined kernel). Works by defining the amount of smooth with named variable `smooth`, something like this:
```julia
plothist2d(hd,range,range,xrange=(-3,3),yrange=(-3,3),smooth=2)
```
and produces this:
![fig2dim](https://f.cloud.github.com/assets/3612029/1524030/f95537de-4bbf-11e3-83c3-8334345887ba.png)

Might be too experimental feature but dunno, merge if you want.