Mese fence

This is a improvement of mese_fence. https://github.com/Jeija/minetest-mod-mesecons/pull/180

Mese Fences can be used as a vertical mese wire with the advantage that mesecons can be connected to the whole extension of it, also, they look batter in several cases.

![screenshot_3657267398](https://cloud.githubusercontent.com/assets/6201512/5157724/71daec0c-7316-11e4-8100-fbebb1975e52.png)
![screenshot_3657309150](https://cloud.githubusercontent.com/assets/6201512/5157727/83a47930-7316-11e4-984a-d86c76b8f6ea.png)
![screenshot_3657350390](https://cloud.githubusercontent.com/assets/6201512/5157732/945d14bc-7316-11e4-86b6-5ce043311c7e.png)
![screenshot_3657361335](https://cloud.githubusercontent.com/assets/6201512/5157729/9325d1ba-7316-11e4-9fda-2aec843a6f90.png)
![screenshot_3657538796](https://cloud.githubusercontent.com/assets/6201512/5157731/93aa6236-7316-11e4-9b78-de7a09726d83.png)
![screenshot_3657614328](https://cloud.githubusercontent.com/assets/6201512/5157737/f46daff6-7316-11e4-8c96-60bde0ee87f4.png)