This has the additional benefit of making it appear in the `rake -T`
output.

Provide two ways of accessing the task. One saves a character (":") and
is reminiscent of the Django command that this project is inspired by.
The other is more Rails-like and is consistent with the other
`db:`-prefixed Rails Rake tasks.

Update our class names to reflect the new file structure introduced in
b700c1bef89a67a64f1040fb6bb03c0320eefe91.

Use the

    module DBShell
      module Client
      end
    end

syntax intead of `DBShell::Client` so that we don't have to define the
`Client` module in a separate file and require it. Otherwise we'd get an
error:

    lib/dbshell/client/sqlite3.rb:1:in `<top (required)>': uninitialized constant DBShell::Client (NameError)

Change the paths in the code to conform to the new file layout
introduced in b700c1bef89a67a64f1040fb6bb03c0320eefe91.

No longer have a use for this information as I've already leveraged in
the database client class implementations.

Instead of calling `DBShell::Sqlite3Client` directly, call the wrapper
class that will delegate to `Sqlite3Client`, `PostgresClient`, etc.
depending on the value of `adapter` in the database connection params
passed in.

This allows us to have a single interface that we can call from the Rake
task that can handle any of the database adapters that we support.

Remove our hard-coded "development" environment and instead get the
current environment dynamically via the `RAILS_ENV` environment
variable. Assume that the current environment is "development" if the
env variable is unset.

Add `DBShell::Sqlite3.runshell`, which takes database connection
information and starts a SQLite3 shell.

Currently the Rake task just calls the SQLite3 `.runshell` method
directly without any database adapter discrimination and also forces the
development environment. It's nice to see it working. Now, we'll need to
clean up those parts of the Rakefile.

Create a stub for our Rake task to see something working.

Inspired by
https://github.com/paulelliott/fabrication/blob/b470cd817e1973c14e3253cd76606cf934942ca1/lib/fabrication.rb