support export as PDF file from File menu
Added word count
Implement method to get text content in DOM tree

Based on jQuery's implementation.
https://github.com/jquery/jquery/blob/bcca4f0/external/sizzle/dist/sizzle.js#L915

This is actually pretty fast, and should be efficient enough.
Use rendered HTML for word count
Base character counts off rendered HTML
Switch wordCount to NSPopUpButton
Fade wordCount when cursor is intersecting
Start with last used MPWordCountType
Experimental front-matter support

The current implementation just rendered the front matter as a
code block.

Outstanding issues:

* Should I try to render the front matter into a table? Need to
  understand more about this; if it'll always be as simple as
  shown in the examples, I can just parse it; if we REALLY need
  to parse it as YAML then we might have a problem here.

* If we don't/can't render it into a table, we should at least
  syntax-highlight it if we're allowed. Maybe I should submit a
  YAML highlighter to Prism?

* The PEG syntax highlighter needs fixing. We should probably do
  it in the highlighter, not the PEG grammar.
Implement object to HTML table rendering
YAML => table rendering

Is there a way to make the keys ORDERED? Maybe we need to drop
back to LibYAML with the Objective-C wrapper. :(
Customize YAML.framework to use ordered dicts
Make setext title 2 require FOUR dashes
Add licenses

LibYAML (MIT)
M13OrderedDictionary (MIT)
YAML.framework (MIT)
Disable word count alpha
Implement padding by resizing scrollview
Cleanup and correction

General code cleanup.
Word count text formatting is now handled by JJPluralForm so that they
can be localised correctly in the future.
Add license for JJPluralForm
Merge branch 'word-count-experimental'

Conflicts:
	MacDown/Code/Document/MPDocument.m
Add Joseph Stevens (@Mowsh) as contributor