Turns out the port actually did need to be a string, contrary to what I
had assumed in e89275bc787ae17bc698ac8cdb8cdaf5ebeb7852. I ended up with
this error when passing a specific port in a real test:

    $ bake dbshell
    rake aborted!
    TypeError: no implicit conversion of Fixnum into String
    .../lib/dbshell/client/postgres.rb:8:in `exec'
    .../lib/dbshell/client/postgres.rb:8:in `runshell'
    .../lib/dbshell/client/database.rb:24:in `runshell'
    .../lib/dbshell/rails/tasks/dbshell.rake:6:in `block in <top (required)>'
    .../versions/2.3.3/bin/bundle:22:in `load'
    .../versions/2.3.3/bin/bundle:22:in `<main>'
    Tasks: TOP => dbshell

PostGIS is, for our purposes, no different from Postgres. Both should
launch `psql`.

* Create a `.runshell()` method in `DBShell::Client::MySQL`
* Delegate to that class & method from `DBShell::Client::Database`

Tried this out with MySQL, and got this error since I haven't integrated
it yet. It struck me that "Invalid" doesn't make any sense. Really it's
something that is possible to do but that our gem doesn't support, so we
should be clearer about that fact.

For now only includes a `.build_command()` method.

Inspired again by:
https://github.com/django/django/blob/66150f7cf61bc09547fa98586790df596eff6d77/django/db/backends/mysql/client.py

although in our case we haven't handled some of the more exotic options.
Eventually we'll probably have to do that.

To match the 'sqlite3' gem's and the SQLite project's capitalisation
convention.

Now that we have a `Client` module
(46b019abd520870ab159b60c03c45675f64dd88a), it makes so much more sense
to put this error class inside it.

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.

Will be modifying the class names in the next commit. Here we change the
file names & locations of the database client files.

The format changes thusly:

    <adapter>_client.rb -> client/<adapter>.rb

This organisation feels cleaner to me. Wasn't liking the previous
structure.