Bandcamp: Switch to esc_attr instead of int for album and track IDs

Bandcamp Album and track IDs can be larger than `PHP_INT_MAX`, thus causing issues in embeds.

View original trac ticket here:
http://plugins.trac.wordpress.org/ticket/2051
Bandcamp shortcode: check if track and album IDs are numbers

See #55
Add Markdown module

* Duplicates support as on WordPress.com
* No need to activate for posting - activating the module is enough.
* off by default; no connection required
Merge pull request #69 from mattwiebe/markdown

Add Markdown module
Widgets: Shortcodes: Add 'Upcoming Events'
Load in the icalendar library that can be included from the widget or the shortcode.  Then add the shortcode and the widget.
Props: @ethitter @jacklenox @yoavf @cfinke @mattwiebe @briancolinger
Tiled Galleries: add alt attributes to images

Original trac ticket:
https://plugins.trac.wordpress.org/ticket/1815
Merge pull request #55 from jeherve/2051-trac

Bandcamp: Switch to esc_attr instead of int for album and track IDs
Merge pull request #72 from jeherve/1815-trac

Tiled Galleries: add alt attributes to images
Merge pull request #74 from georgestephanis/upcoming-events

Widgets: Shortcodes: Add 'Upcoming Events'
Widget: Upcoming Events: Add (Jetpack) to title

Add the jetpack_widget_name filter, and space out the
parent::construct() call a bit for legibility.
See #74 props @jeherve