show past talks

Couple things:
There are 4 tests in the current test suite fail, since some methods in the TalksController and model were renamed/modified. As I am not familiar with minitest, I was hesitant to change the test files, so I wrote some some tests in rspec instead which address the failing tests.  They live on rspec branch of the forked repo.

To avoid any possible conflict with testing and since I had so much trouble with gem dependencies, I did not actually merge the rspec branch with my master branch.  The tests do pass, however--I'm just not sure how to convert them into minitest syntax.

The previous, closed pull request had two additional features of note: 1) fixing the Twitter API call to fetch profile pictures and 2) rendering the talk submission form as a modal.  It turns out I couldn't gracefully resolve the dependency issues with the gems I needed to implement these features, so they're unfortunately absent from this pull request.