Add default client, accessible through TempoDB module

I've added the ability to use a default client in applications which have only one database and a single set of keys. In one particular use case, in a Rails initializer, one would be able to use:

```ruby
TempoDB.configure do |config|
  config.database_id = ENV['TEMPODB_DATABASE_ID']
  config.api_key     = ENV['TEMPODB_API_KEY']
  config.api_secret  = ENV['TEMPODB_API_SECRET']
end
```

Standard `TempoDB::Client` instance methods can then be called as class methods of `TempoDB`:

```ruby
TempoDB.write_multi(a_bunch_of_data_points)
```