Fix for #4 -- allow callbacks in `run()`

Consumers can now include a callback in a query's `run()` to allow for a subscription type API for partial or feed type responses.

**Example**
```perl
use feature ':5.16';
use Data::Dumper;
use Rethinkdb;

# setup
r->connect->repl;
r->db('test')->create->run;
r->db('test')->table_create('table')->run;

# receive changes
r->db('test')->table('table')->changes->run(sub {
  my $changes = shift;
  if( @{$changes->response} ) {
    say {*STDOUT} Dumper($changes);
  }
});

# force the script to stay open
while(1) {}
```