Unit tests

[reiinakano]

Now that Xcessiv is in a (sort of) stable state, it's time to correct my bad practice and start writing comprehensive unit tests. Untested code is broken code.

If anybody wants to help out, that would be very awesome! If you're a React.js dev willing to write tests for the JS part of Xcessiv, that's extra awesome! This was my first ever project using Javascript and I have a lot to learn in that direction.