Switch from jasmine to jest (#747)

Summary:
The reasons for switching to jest:
- easy snapshot testing so that we can easily verify the structure of the parse tree and MathML tree
- easy compilation of ES6 features for tests as we continue to expand our use of ES6

Test Plan:
- npm test

.gitignore

@@ -12,3 +12,4 @@ diff.png
 /test/symgroups.log
 /test/symgroups.pdf
 /test/screenshotter/unicode-fonts
+coverage/