Set default cookie path for MetaProtoExtendedSession to contextPath

[Debilski]

As suggested in https://groups.google.com/d/msg/liftweb/ZWmPdpIG1JA/dfAzbXiTBgAJ the cookie path for MetaProtoExtendedSession should be configurable and/or default to contextPath.

[Shadowfiend]

This should be as simple as updating https://github.com/lift/framework/blob/master/persistence/mapper/src/main/scala/net/liftweb/mapper/ProtoExtendedSession.scala#L95 to use S.contextPath instead of "/", I think. However, this would be a breaking change.

As such, we probably need to make this customizable instead. I think we can add a:

  def sessionCookiePath: String = "/"

To the trait. We can even add a new trait, ContextPathExtendedCookie, that sets the above to S.contextPath, so you could do extends MetaProtoExtendedSession with ContextPathExtendedCookie or something like that.