Adding Phone To helper like mail_to for handling phone numbers

Adding Phone To helper like mail_to for handling phone numbers

Useful for mobile browsers and desktop browser with phone supported software.

Creates a tel link tag to the specified +phone_number+

Accepts blocks

I was not able to achieve this
I started out by doing it, Trouble is we need to only append tel: to the href options not to the link Text so it is to be viewed similar to mail_to?

If i override href before i call link_to for the odd test case someone wants 
```html
<a href="tel:1234">tel:1234</a>
```
it will become 
```html
<a href="tel:tel:1234">tel:1234</a>
```

Please advise unless i am missing something