Terminal make raw

Allow better interactive mode. Code inspired by what is done in the `terminal` package from go `crypto/ssh` (see source code [here](https://code.google.com/p/go/source/browse/ssh/?repo=crypto#ssh%2Fterminal)). 

Basically:
* a function to make the tty raw, i.e disabling the echo when stdin is redirected to the tty among other things
* a function to restore the state of the tty
* a new `StartRaw` function that allows to use the improvement listed above without changing the existing API

I added an example of how to use it in the `README.md`.