Add minimal implementation of firefox profile.

[danhper]

This is a very minimal implementation of Firefox profile.
It is enough to set the user agent, but we may need to
add at least some utility to load an existing profile.

The usage looks like this:

alias Hound.Firefox.Profile

profile = 
  Profile.new
  |> Profile.set_user_agent("my_ua_string")
  |> Profile.put_preference("some.key", "some-value")
  |> Profile.dump
Hound.start_session(%{firefox_profile: profile})

I am not sure if there is some preferences we should add to default_prefs,
if anyone has any idea, let me know.
Also, I am not sure if it is ok to have everything in user.js or if we really need
to make a difference between prefs.js and user.js, any idea?

/cc @darksheik @HashNuke