Allow histogram.py and other scripts to be imported/used as python modules

[bw2]

Hi, I'd like to generate histograms to standard out in my python script, and currently have to do
os.system("echo '%s' | historgram.py" % "\n".join(values))

It would be great if I could instead do

from data_stacks import histogram
histogram.histogram(values)

[bsergean]

I'd like to have that too ... I don't think it would be too hard to make a pull request for that I should do that.

[pylipp]

You should check out #41, it's pretty handy.

[jkawamoto]

Thanks for referring the PR. I forgot to mention this issue from that PR.