IDatabaseClient should implement IDisposable

[jiri-matejka]

Interface IDatabaseClient should implement IDisposable because its implementation, DatabaseWrapper, is disposable:

internal class DatabaseWrapper : IDatabaseClient, IDisposable

Currently I cannot use using statement since IDatabaseClient returned from await DatabaseClient.Create(...) is not disposable.

The only thing what I can do is

var dbClient = await DatabaseClient.Create();
try
{
   await dbClient.Execute(...);
}
finally
{
  ((IDisposable)dbClient).Dispose();
}

[tvandinther]

Fixed in #30