Support Oracle 12c top-N query 

This pull request has two separate commits to support Oracle 12c better Top N query support using `FETCH FIRST n ROWS ONLY` and `OFFSET n ROWS`.


8f5e57f8c3a0d497cdbd153c7cd59f0168c858c4 Extract visit_Arel_Nodes_SelectOptions

It extracts `visit_Arel_Nodes_SelectOptions` from `visit_Arel_Nodes_SelectStatement` method to prepare Oracle 12c support since it will be able to reuse most of super's `visit_Arel_Nodes_SelectStatement` method because Oracle requires to add collector in the following order.
```
offset
limit
lock
```
Unlike other database require in this order.
```
limit
offset
lock
```

This method is not called using visitor pattern, I'd like to hear `visit_Arel_Nodes_SelectOptions` is an appropreate method name.

371b64268ee2ec39b13cf347f2723b7aa144c3a1 Create Arel::Visitors::Oracle12 to provide better top-N query

Oracle Database 12c introduces better top-N query support using `FETCH FIRST n ROWS ONLY` and `OFFSET n ROWS`.  Also to support older version of Oracle 11gR2 or lower version, this commit creates
another visitor named `Arel::Visitors::Oracle12`, 12 comes from the database version.

By changing oracle enhanced adapter as follows to execute each Arel visitor based on
if this top-n query feature supported. https://github.com/yahonda/oracle-enhanced/commit/fcb22dbae0cd6f5baf8c68c57cff8d2c9f5b7c18

```ruby
if supports_fetch_first_n_rows_and_offset?
  @visitor = Arel::Visitors::Oracle12.new self
else
  @visitor = Arel::Visitors::Oracle.new self
end
```

It looks `Arel::Visitors` just allows 1 database adapter to have 1 visitor, but my implementation works to switch two visitors just changing `Arel::Visitors::Oracle12` or `Arel::Visitors::Oracle` based on its database version. It looks 1 database adapter can have 2 or more visitor(s).  Are these lines can be removed? 
https://github.com/yahonda/arel/blob/a19190ab06db4d68765fb08ec1a4dc6c8ebb2eb5/lib/arel/visitors.rb#L15-L41

```ruby

module Arel
  module Visitors
    VISITORS = {
      'postgresql'      => Arel::Visitors::PostgreSQL,
      'mysql'           => Arel::Visitors::MySQL,
      'mysql2'          => Arel::Visitors::MySQL,
      'mssql'           => Arel::Visitors::MSSQL,
      'sqlserver'       => Arel::Visitors::MSSQL,
      'oracle_enhanced' => Arel::Visitors::Oracle,
      'sqlite'          => Arel::Visitors::SQLite,
      'sqlite3'         => Arel::Visitors::SQLite,
      'ibm_db'          => Arel::Visitors::IBM_DB,
      'informix'        => Arel::Visitors::Informix,
    }

    ENGINE_VISITORS = Hash.new do |hash, engine|
      pool         = engine.connection_pool
      adapter      = pool.spec.config[:adapter]
      hash[engine] = (VISITORS[adapter] || Visitors::ToSql).new(engine)
    end

    def self.visitor_for engine
      ENGINE_VISITORS[engine]
    end
    class << self; alias :for :visitor_for; end
  end
end
```