Load Rails, which will load teaspoon_env

[mikepack]

If we always load Rails, we can assume that teaspoon_env.rb will get loaded as part of the engine initialization. This doesn't help us move away from Rails, but it's the simplest way to ensure that teaspoon_env.rb is only being required once, without limiting any behavior.

Additional:

• :+1: Removes the needs for teaspoon_env.rb to load the Rails environment which I think is cleaner. If we want to go the direction of this PR, I propose we rename this file to teaspoon.rb.
• :-1: We now need to know how to boot Rails. I've added the TEASPOON_RAILS_ENV env var to make this overridable. Works with a stock setup, but is required every time we run teaspoon tests using the dummy app's environment.rb.
• :-1: Ties us even closer to Rails.
• :+1: Is less code than my previous approach #340 (which I've since reverted in master).

Fixes #341

[jejacks0n]

I'm not opposed to this, but let me screw around with it to see how easy it would be to take rails out as a dependency for a bit. It kinda plays into supporting middleman, and potentially providing a simple rack middleware layer that teaspoon uses if rails isn't present -- though that sounds particularly unpleasant. heh.