Added OpenGraph tag & namespace injection
Fixed multiple OpenGraph namespace injection

OpenGraph namespaces as html tag attributes should now only appear once
Adjusted OpenGraph namespace injection priority

A lower priority will minimize plugin conflicts when injecting html
attributes.
Sanitized og:description tag

get_the_excerpt() is not run through htmlspecialchars() before display -
added a call to this before outputting to the og:description tag.
Updating version number

Updating version to 1.3.0 for merge into master
Merge pull request #3 from ContentLEAD/dev-1.3

Merging in dev changes for v1.3.0