disable opengraph support in jetpack

fixes #11
update readme and bump to v1.5
version bump
disable jetpack's opengraph functionality

hopefully for real this time.  In theory, the 'jetpack_enable_opengraph'
filter should have been sufficient, as it prevents jetpack's
functions.opengraph.php file from being loaded at all.  However, since
it is being called at plugin load time (rather than as part of the 'wp'
action for instance), it's possible that it is called before our
opengraph.php is even loaded.  Fortunately jetpack also has a
'jetpack_enable_open_graph' (note the underscore) filter that is called
inside their jetpack_og_tags() function.

fixes #13
prep for tagging 1.5.1 release
version bump
Merge branch 'master' of git://github.com/willnorris/wordpress-opengraph
structured post format support

added support for the "structured post format" support coming with
WordPress 3.6