Add __getitem__ support to connections and buckets

[jgeewax]

[tseaver]

Duplicate of #29 (for connections and buckets). I don't think __getitem__ has sensible semantics for keys, given that we are exposing the representation's properties via accessors/mutators in #313.

[jgeewax]

So just to clarify:

bucket = connection['bucket-name']
object = bucket['object-name']
property_value = object['meta-data-field-name']  #  <-- You're saying this is the one we don't really care about, right?

If we're exposing these meta-data properties via methods, then I'm fine with the conclusion. Might want to update the title of the bug to specify what we're actually "closing" here though, right?

[tseaver]

Yes, I meant that the object's properties ('contentType', 'etag', etc.) will be exposed via methods or Python properties. #349 is where they actually got implemented.

I updated the title.