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.
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
Merge pull request #1574 from jlduran/latex-horizontal-rule

LaTeX writer: Make Horizontal Rules more flexible
Updated changelog.
DokuWiki writer: Use backslash newlines in table cells.

Write out strings in table cells with backslash linebreaks in place of
newlines. We also want to remove the first two spaces of an indent in lists.
Removed extra blank line after version.
Merge pull request #1580 from jkr/stringCellDokuWiki

DokuWiki writer: Backslash newlines in table cells
Updated README on `--self-contained`.

Removed outdated claim that pandoc will look in the user data
directory if a relative path is specified and the file is not
found locally.  Closes #1572.
Updated changelog.
Merge branch 'mime' of https://github.com/Aelve/John into Aelve-mime

Conflicts:
	src/Text/Pandoc/Writers/Docx.hs
Updated changelog.
Dokuwiki writer: Add a test for multiblock table cells.

We have to add a new file, because the original table tests don't look
for this.