Move deployment support to separate gem

Most pull requests to dotenv are to support different deployment scenarios. As I explained in #71, I'm not convinced that dotenv should even be used in production. But I also live in a fantasy land where I don't have to manage production environments. Either way, I'm not the right person to be maintaining features that I don't need or use.

So here's my proposal: move all deployment related stuff to [dotenv-deployment](https://github.com/bkeepers/dotenv-deployment). dotenv will remain a very small shim intended for development. Someone else can manage dotenv-deployment and let it evolve into whatever people need for managing production configuration.

I would love thoughts from people that don't live in my fantasy land. /cc @wfarr @jnewland @asenchi @garno @elskwid @markrebec @linyows

If this sounds like a good idea, then I would love help testing these changes.

```ruby
gem 'dotenv', :groups => [:development, :test], :github => 'bkeepers/dotenv', :branch => 'deployment'
gem 'dotenv-deployment', :github => 'bkeepers/dotenv-deployment'
```

Once everything appears to be working, then I will:

* Release 0.11.0 with a dependency on dotenv-deployment and deprecation warnings for moved features
* Remove deprecation warnings and dependency and release 1.0

Closes #71