import unit test, more use cases for num_databases

Per Jeff's suggestion, adding a unit test for 'use $class num_databases => 21'.

Adding a helpful error message if you try to select() past the number of
databases that you've got configured--should the error message be the same
as redis's "ERR invalid DB index"?

Also expanding it so you can do $class->new(num_databases => 21)--it's not my
use case but I can see where it would come up.
more use cases for num_databases

Adding a helpful error message if you try to select() past the number of
databases that you've got configured--should the error message be the same
as redis's "ERR invalid DB index"?

Also expanding it so you can do $class->new(num_databases => 21)--it's not my
use case but I can see where it would come up.

(should have done "git commit -a" on previous commit)
making third test more obvious
remove num_databases as an argument to new()

Because it's not part of Redis.pm, which we're mocking.

I added a class method so it can be changed at runtime without
having to call $class->import
don't need args to _defaults() any more
more cleanup to match code in master branch
Merge pull request #10 from kgoess/master

ability to change number of available databases