Added EPUB Reader

EPUB Reader.

Note: If you want to use this patch you also need 41f1d48 and the development verion of texmath (0.7)

Supported features

1. MathML parsing
2. epub:switch with support for reading MathML in cases
3. epub:type attributes are either passed as styles or native support is included for footnote, endnote, noteref, toc and chapter.
4. All image paths are scrubbed the the bare filename, if you want images then for now you have to unzip the epub yourself. We are working on a solution to this.

Problems:

1. I think that the internal referencing now works properly but the implementation could be improved with a clearer idea of what normalisation is required.
2. Images
3. EPUB has a lot of features, there are doubtlessly things which could be improved upon in my implementation or approximated better within the pandoc AST. Please comment if I have missed something!
4. Tests could be improved.

Closes #652