Adds "object" field for Activity
Removes "provider" argument from `makeActivity`

According to the spec, the only required fields are "actor" and "published".
And I don't really have anything to put in the provider field yet.
Adds `asObject` function.

The spec says that an Activity may be used as on Object.  This adds a function
that just does a type cast to support that.
Adds LensInternal to modules list

I was not able to compile the project without this addition.
Merge pull request #1 from hallettj/pull-requests

PR looks good to me. (As a semi-relevant aside: I might eventually remove the non-Dynamic module and make Dynamic the primary way of interacting with the library. I hope that would make sense!)