MediaBag: Fixes Windows specific path problems

Changes the internal representation to fix the problem.

I haven't tested this on windows.

Closes #1597
Shared: Make collapseFilePath OS-agnostic
EPUB Tests: Seperating image testing from other features
HTML Writer: MathML now outputted with tex annotation.

Closes #1635
Add support for KaTeX HTML math

Closes #1626
Updated README to include information about KaTeX switches
HTML Writer: Don't double render when email-obfuscation=none

Closes #1625
HTML Reader: Recognise <br> tags inside <pre> blocks

Closes #1620
Org Reader: Parse multi-inline terms correctly in definition list

Closes #1649
Update tests for #1649
Merge pull request #1644 from mpickering/master

Fixes and Enhancements from the tracker