Add element reference operator #[] to Mysql2::Result using mysql_data_seek

[sodabrew]

For #130 mysql_data_seek is wrapped behind the element reference operator #[]. You can jump directly to a particular result row:

  result = @client.query "SELECT 1 UNION SELECT 2"
  p result[1]

  {"1" => 2}

Open for comment!

[sodabrew]

The fourth changeset builds on the #[] changeset, adding support for:

  result = @client.query "SELECT 1 AS col UNION SELECT 2 AS col"
  p result[1, 0]
  []

  p result[1, 1]
  [{"col" => 2}]

  p result[-2, 2]
  [{"col" => 1}, {"col" => 2}]

  p result[1, {:symbolize_keys => true}]
  {:col => 2}

  p result[1, 1, {:symbolize_keys => true}]
  [{:col => 2}]

Now that I'm returning an array, I should probably support cacheRows, which means the code in result#[] is going to look a lot like result#each. More refactoring!

[sodabrew]

Two years later - rebased yo!

[tamird]

underindented by one?

[tamird]

this preamble could be implemented in ruby and would be much easier to read - why not do that?

[sodabrew]

I actually moved this further down the chain so that the same code is used by the variants of [] and each.