Add 3rd party library highlight.js

Add 3rd party javascript library highlight.js
(http://softwaremaniacs.org/soft/highlight/en/). It is used for
highlighting syntax in code blocks.
Include script and styles in resource file

Include used script and styles from library highlight.js in resource file
resources.qrc.
Change links to highlight.js

Instead of linking to the online version of highlight.js, we use the
script and styles in the resource file resources.qrc.

This way the code highlighting works without internet connection.

Resolves issue #55.
Move HTML formatting of TOC

Move the HTML formatting of the table of contents from MainWindow to the
HtmlPreviewGenerator class.
Add more documentation to source code
Changed string "Application" to "Save Changes"

issue #44
Remember math support and code highlighting options

Extend Options class with handling of the state of math support and code
highlighting option.

Resolves issue #54
Merge branch 'release/v0.7.2' into develop
Merge branch 'feature/offline-highlighting-#55' into develop
Merge branch 'feature/issue-#54' into develop

Conflicts:
	app/htmlpreviewgenerator.cpp
New Insert Table dialog

Added a new dialog to define the size, alignment and content
of a table.

Issue #51
Add method insertTable to MarkdownManipulator

Add new method insertTable() to MarkdownManipulator class. This method
inserts a markdown table defined with the TableToolDialog.

Issue #51
New menu item "Insert Table..."

Add new action "Insert Table..." to edit menu. It shows the
TableToolDialog and uses the MarkdownManipulator class to insert a table
into the markdown document.

Issue #51
Merge branch 'feature/table-tool-#51' into develop
Update discount submodule
Fix encoding problems in table of contents

Fix the display of german umlaut characters in the table of contents dock
widget.

Resolves issue #70
Fix encoding problems in exported HTML

Add <meta> tag to the HTML template to prevent encoding problems with the
exported HTML.

Resolves #71
Add Byword-like style

Add Byword-like style from https://github.com/kvanlierop/byword-for-mou
Change key shortcuts for style entries (fixes #74)

Move style key shortcuts to ui file and change key modifier from Qt::ALT
to Qt::CTRL.
New Insert Image Dialog

Add a new dialog to insert an image link into the markdown document.