set fb_object_id field for Facebook posts

useful for de-duping photo posts and photo objects in bridgy. also add PHOTO object to facebook_test.
update oauth_dropins
update webutil
prefix oauth_callback path with site name

...so i can reuse my one instagram app that has a localhost oauth redirect URL. instagram only lets you set one redirect URL per app, and you only get 5 apps total, so i have to share one app for localhost testing across all of my apps, so they all need the same redirect URL. :/
drop instagram client id/secret usage since we get the access token from oauth-dropins

removes app engine dependency entirely when used as a library
readme: clarify that just the tests require app engine

also drop an unnecessary import in source_test
added timezone to datetime

imported ```pytz``` and added ```UTC``` as the timzone for twitter
added timezone tweet dates

```rfc2822_to_iso8601``` gets timezone support
regex bug fix

fixed regex error in timezone stuff in ```rfc2822_to_iso8601```
fixed some style issues
OffsetTzinfo has default 0 offset
added timezone to all dates being tested
updated Twitter tests with timezones
Merge branch 'master' of https://github.com/kartikprabhu/activitystreams-unofficial