Add ability for users to download content

Add ability for users to download content with CONTENT_SHOW_FILES in config.php. If CONTENT_SHOW_FILES is set to true, then files that reside in the content directory (set via CONTENT_DIRECTORY in config.php) will be displayed on user homepage.

The CONTENT_DIRECTORY path can contain files and n-number of sub-directories.  The files in the sub-directories will be under a section with the name of the sub-directory.  Also, directories will have underscores converted to a space.

![image](https://cloud.githubusercontent.com/assets/3742859/2875420/3ed7ebd8-d41b-11e3-8b66-ffc30885ac59.png)

The file structure used in the screenshot above was the following:

![image](https://cloud.githubusercontent.com/assets/3742859/2875454/3983293a-d41c-11e3-8f5b-6832598c5299.png)