Bug fixes

Closes undefined variable issue #60
Checks for MarkdownExtra class before loading it
Don't show PHP errors if HTML is invalid (we fix it anyway)

Closes #58
Merge pull request #59 from nextgenthemes/master

Programatic approach for creating this many shortcodes
Clean up shortcode functions working from @nextgenthemes' example
Clean up row, column
Clean up shortcodes

Clean up the following:
* List group
* List group item
* List group item heading
* List group item text
Manually merge @nextgenthemes’ re-try at Nav Item
Shortcode cleanup

Cleaned up:
* Breadcrumb
* Breadcrumb item
* Label
* Badge
* Icon
* Well
* Panel (Bonus: panel title no longer required)
This is a breaking changes. Allow panels with or without <h3> titles

This renames one of the parameters for panels: former “title” is now
“heading”, and “title” is a boolean which applies a title tag to the
heading or not.
Fix backwards compatibility for Panels

Panels now check whether the value of $title is a boolean value, if not,
it sets $heading to $title and $title to true.
Boolean check didn't work, removed. Fixed panel-body.
Begin work on scrollspy
Beginnings of scrollspy support
Thought better of scrollspy
Clean up tabs, add shortcode parser function

Added a function to step through nested shortcodes and retrieve their
properties.
Indentation and readability
Clean up collapse
Fade, active, pills style parameters for tabs
@nextgenthemes' parse_data_attributes
Attempted fix of file paths on Windows
Attempted fix for file paths in Windows