Added {thumbnail} and {featuredimage} template tags

Added two tags for use in the publish format template:
{thumbnail} grabs the featured image thumbnail for the post
{featuredimage} grabs the full-size featured image for the post

Note: Using {featuredimage} may or may not need to have "create_thumbnails" enabled in discourse for best results

An example of how this comes out can be seen here:

http://forums.ctrlcmdesc.com/t/costume-quest/298
Allow for custom excerpts

Our site uses a custom excerpt format, so this allows anybody to override the excerpt by simply defining a function in their own code.
Merge pull request #31 from howtogeek/patch-1

Allow for custom excerpts
Merge pull request #29 from surrealroad/patch-6

Added {thumbnail} and {featuredimage} template tags
added auto track option that allows authors not to automatically
track topics that are posted on their behalf

added a lock around sync to discourse to ensure double posts don't trigger dupes
don't wait for the lock, cause trouble ...
update version to 0.5.0
add additional author
update copyright
update readme
add image to readme
clarify comments
Add a settings field for the excerpt length
added function
Merge pull request #36 from surrealroad/patch-8

Add a settings field for the excerpt length
Fix incorrect parameter
Merge pull request #37 from ly/patch-1

Fix incorrect parameter
Removed timestamp call that required PHP 5.3
Merge pull request #40 from freney/master

Removed timestamp call that required PHP 5.3
don't allow non-authenticated users access to options