Add mouse features

[yingqiuz]

add mouse module, including:
(1) retrieve gene info, do gene id <->name/acronym conversions, check gene validity
(2) retrieve structure info, do structure id <->name/acronym conversions, check structure validity
(3) fetch unionisation data, including expression-energy, expression-density, sum-pixels, etc.

[rmarkello]

Hi @yingqiuz ! It looks like the TravisCI is failing because of a dependency issue. I've updated the master branch to fix this so merging it into your PR should fix the issue right up!

Let me know if you have any questions about how to do that! The first answer to this StackOverflow question should do the trick 🙂

[rmarkello]

Hi @yingqiuz ! I wanted to let you know that I haven't forgotten about this but I have just been super bogged down by other things. I will try and get to reviewing this soon ! Apologies for the delay, and thank you again for this awesome contribution ✨ 🙌

[rmarkello]

The Travis is definitely going to fail, but I'm working on refactoring some things and will address the tests as soon as I'm done!

[rmarkello]

The coverage decreased a bit, but the tests are passing! I'm going to go ahead and merge this in now, @yingqiuz. Thanks so much for your work!