Added uri option handling.

[rwz]

Heroku usecase:

use Rack::GridFS, prefix: 'gridfs', uri: ENV['MONGOLAB_URI']

[ches]

Does from_uri also allow you to pass a URI without inline user/password included, and then call authenticate similarly to connection by enumerated options? If so it would be nice to support that here. That's my only feedback, otherwise I think this pull request looks good.

[max-power]

see docs: http://api.mongodb.org/ruby/current/Mongo/Connection.html#from_uri-class_method

[ches]

My question was semi-rhetorical 😉 The answer is indeed "yes."

The value of the option also ought to default to ENV['MONGODB_URI'].

[rwz]

MONGODB_URI is not the only available key. For example if you use MongoLab addon on heroku, it'll be MONGOLAB_URI.

[ethnt]

Is this being pulled in at some point? This would be a nice feature to have.