Two minor improvements - findByTag and hidden type support like android

I like CSLinearLayoutView very much. It is difficult to arrange the views in iOS without it.

I have two ideas to make life easier.

1. add findByTag for and let the CSLinearLayoutItem share same tag value with its view.
https://github.com/stonyw/CSLinearLayoutView/commit/a177cb4aca3fe4362bae78c872c0cb561c10248c
https://github.com/stonyw/CSLinearLayoutView/commit/41656a5f1671885527c2dbc6aa62219200002f29
I saw CSLinearLayoutItem did have tag and didn't use it yet. I added findByTag first. I find it is hard to maintain two tag id lists if I use tag. It is better to share the same tag id list.

2. add hidden support.
https://github.com/stonyw/CSLinearLayoutView/commit/09d8b9c3def4e9f98156e6760f5d8a7367eaee20
Hidden has two meanings in Android: Invisible and Gone. I want to hide some views in CSLinearLayout and show them again later. "Gone" is useful to me. 

3. The rest commits are for ARC. @jaydee3 has a pending pull request https://github.com/scalessec/CSLinearLayoutView/pull/2 .