Pr/save ascii

Howdy,

So long, but still fighting matrices (unfortunately not petting ducks :-) ).
Now for the real matter, I found it would be nice to have methods to save/load matrices from files.
I know I can slurp a file and use 'new_from_string', but I do not think that to be very efficient (as you need to construct the full string in memory, and then parse it). The same to save...

So, I know I miss tests and documentation. But for now what I want is:
   * if you bless the creation of these methods
   * if you want to change the methods names, or if I can keep them

If everything is good, I'll continue coding, and will poke you again when it is ready.

Cheers