MIME cleanup.

  * Create a type synonym for MIME type (instead of `String`).
  * Add `getMimeTypeDef` function.
  * Avoid recreating MIME type `Map`s every time.
  * Move “Formula-...” case handling into `getMimeType`.
Update Reader.EPUB to use `MimeType`.
Remove an unnecessary import.
Docx writer: Implement track changes.

These have default authors and dates of "unknown" and timestamp-zero,
respectively.
Docx writer: Default to user login and time of change if not given.
Docx writer: Accomodate GHC 7.4 (no lookupEnv)
Fixed exampleLine parser to accept example lines which have indentation at the start of the line.
Merge pull request #1565 from Chobbes/master

Org exampleLine parser accepts indented example lines!
Merge pull request #1564 from jkr/trackChangesWriter

Docx writer: write track changes.
DokuWiki Writer: Qualified all imports
DokuWiki Writer: Hlint cleanup
DokuWiki Writer: Refactor to use Reader monad
Merge pull request #1567 from mpickering/dokuwidth

DokuWiki cleanup
LaTeX writer: Fix tables

- [x] Fix a bug introduced in 66378062b622b0815a1a2ddce5d557e3ad13330c, which
  causes the table caption to repeat across all pages
- [x] Address the issues discussed
  [here](https://groups.google.com/forum/#!msg/pandoc-discuss/qMu6_5lYy0o/ZAU7lzAIKw0J)
  regarding the extra vertical space.
  - [ ] NOTE: This will cause multiline table cells to appear unpadded. See
    http://tex.stackexchange.com/questions/34971
  - [x] Use [`\tabularnewline`](http://tex.stackexchange.com/questions/78796)
    instead of `\\`.
LaTeX writer: Make Horizontal Rules more flexible

Currently, pandoc has hard-coded the following in order to make horizontal
rules in LaTeX:

```hs
"\\begin{center}\\rule{3in}{0.4pt}\\end{center}"
```

Which is fine, but does not allow customizations.  It also does not take into
consideration the current line width.

I'm proposing this change:

```diff
@@ In Writers/LaTeX.hs:
-"\\begin{center}\\rule{3in}{0.4pt}\\end{center}"
+"\\begin{center}\\rule{0.5\\linewidth}{\\linethickness}\\end{center}"
```
Docx Reader: Read single para in table cell as plain

This makes to docx reader's native output fit with the way the markdown
reader understands its markdown output. Ie, as far as table cells go:

docx -> native == docx -> native -> markdown -> native

(This identity isn't true for other things outside of table cells, of
course).
Docx reader: update tests for new table behavior.
Merge pull request #1577 from jkr/plainCells

Plain cells
LaTeX writer: Add `\strut` to fix multiline tables

See: http://tex.stackexchange.com/questions/34971
Merge branch 'ugly-tables' of https://github.com/jlduran/pandoc into jlduran-ugly-tables