Add support for multiple connections

You can now configure multiple connections like so:

    use OmniAuth::Strategies::LDAP, 
      :title => "My LDAP", 
      :host => ['10.101.10.1', '10.101.10.2'],
      :port => [389, 389],
      :method => [:plain, :plain],
      :base => ['ou=int1, dc=intridea, dc=com', 'ou=int2, dc=intridea, dc=com'],
      :uid => ['sAMAccountName', 'uid],
      :name_proc => [nil, Proc.new {|name| name.gsub(/@.*$/,'')}],
      :bind_dn => ['default_bind_dn', 'admin@intridea.com'],
      # Or, alternatively:
      #:filter => [nil, '(&(uid=%{username})(memberOf=cn=myapp-users,ou=groups,dc=example,dc=com))'],
      :password => ['password1', 'password2']

omniauth-ldap will then try each connection in order when trying to authenticate the user.

This feature allows you to search multiple domains/hosts, cases where bind_dn's exist in subtrees and multiple subtrees are needed.

- [x] Rebase to `teambox` branch and reopen PR (old PR here: https://github.com/teambox/omniauth-ldap/pull/3)