Scale images to fit the page for DOCX

Images that are bigger than the page were truncated. This will now
scale them if they are larger than the page. The scale is currently
hardcoded with Word "letter" defaults (page size and margins)
Textile reader:  don't allow inline formatting to extend over newline.

This matches behavior of RedCarpet, avoids some ugly bugs, and improves
performance.
Textile reader:  fixed list parsing bug. Closes #1500.
OMath parser: Change signature of exported function.

This changes the signature of the exported `readOMML` to `String ->
Either String [Exp]`, so it can now, in theory, be slotted into
TeXMath. It doesn't have any real error reporting yet, but that might
make more sense once I put it in a branch, and understand how it works
in the other readers.

It also now reads strings that parse to either oMath or oMathPara
elements. Note that the distinction is lost in the output. It's up to
the caller to remember the display type.
Markdown writer:  Respect -raw_html.

pandoc -t markdown-raw_html should not emit any raw HTML, even
span and div tags that go with pandoc Span and Div elements.

Cleaned up a bit of the logic with extensions and plain.
Merge pull request #1503 from jkr/streamlineMath

OMath parser: Change signature of exported function.
EPUB Reader: Removed incorrectly set reader flag
Shared: Added collapseFilePath function

This function removes intermediate "." and ".." from a path.
Added tests for collapseFilePath
EPUB Reader: Use collapseFilePath
EPUB Reader: Fixed regressions in image extraction

Before the images were relative to the position of the package file. The
collapse function changed this so that they were then absolute in the
archive but the fetchImages function wasn't updated to recognise this.
EPUB Reader: Improved robustness of image extraction

We now maintain the invariant that when fetchImages is called,
all images have absolute paths.

This patch fixes several bugs relating to this as there are three places
where images can be introduced.
  (1) During the HTML parse
  (2) As spine elements
  (3) As a cover image

For (1), the paths are corrected by the transformation renameImages
For (2) and (3), we need to append the "root" to the path we parse from the
spine
Plain writer:  Use ALL CAPS for level 1 headers.
Updated changelog.
Added `native_divs` and `native_spans` extensions.

This allows users to turn off the default pandoc behavior of
parsing contents of div and span tags in markdown and HTML
as native pandoc Div blocks and Span inlines.

Setting of default epub extensions has been moved from the EPUB
reader to Text.Pandoc.
Merge branch 'newbranch' of https://github.com/mpickering/pandoc into mpickering-newbranch

Conflicts:
	src/Text/Pandoc/Readers/EPUB.hs
Updated changelog.
HTML writer:  Don't include empty TOC items for slide shows.

Previously creating a slide with a horizontal rule would result
in an empty list item in the TOC.  This patch fixes that.
EPUB Reader: Changed image paths to be relative to manifest file
Updated EPUB tests