Imports `os` module in the beginning of the file 

The module is used to check if the font exists, but it was only imported if wordcloud.py was run as a script. This meant calling make_wordcloud from another module failed.
Adds requirements.txt
Changes default font to DejaVuSansMono

This font seems to be more commonly available than DroidSans
Extracts showing and saving the image from make_wordcloud

The function now only creates the Image object, and returns it so the caller
can do whatever she wants with it.