Make model repo configurable for read/write

[paulcsmith]

Sometimes people want to write a master and read from one or more replicated database for performance reasons.

Allowing people to override the repos for reads and writes would not be too hard and would make LuckyRecord more suitable for high performance/high availability requirements.

class MasterRepo < LuckyRecord::BaseRepo
end

class ReadRepoOne < LuckyRecord::BaseRepo
end

class ReadRepoTwo < LuckyRecord::BaseRepo
end

abstract class BaseModel
  def repo_for_reads
     # Choose one of the slave repos to read from
     [ReadRepoOne, ReadRepoTwo].sample
  end

  def repo_for_writes
    MasterRepo # Write to master
  end
end

MasterRepo.configure do
  settings.url = # connect to master
end

[paulcsmith]

Closing favor of #138