Dotenv.parse, Dotenv::Generator and rake dotenv:generate

#### `Dotenv.parse` and `Dotenv.parse!`

Both behave exactly like their `Dotenv.load` and `Dotenv.load!` counterparts, except they do not apply the parsed key/value pairs to the `ENV`.

#### `Dotenv::Generator`

A class which provides an interface to parse example and default-value dotenv files, and programmatically modify the key/value pairs or prompt the user for input. See the README updates to get a pretty good idea of how this works.

#### `rake dotenv:generate`

There are two simple rake tasks wrapped around `Dotenv::Generator` to make generating files for any environment easy. One is generic, while the other is Rails-specific, and overrides the first. The latter simply has the added nicety of using `Rails.env` by default when you do not provide an environment.