add emoji support, with some tests

Hi Mariano,
This is my attempt to add emoji support (#81). Works great for me. It uses my customized version of https://gist.github.com/lepture/2011858
I customized it because the original was missing quite a few emoji. My version pulls a list from Github's API, falling back to a hardcoded version of that list if there is an error, such as Github rate limit being hit (60 requests /hr). With normal usage, the rate limit shouldn't get hit, because snakefire caches it (only hitting the API once every time the program is loaded)
I added some tests of emoji.py for good measure. Let me know what you think!