Add parameter module_size to as_svg export

If parameter is omitted, the default of 11 will be used (no change to current behaviour).
If module_size is used, it allows to render smaller (or larger) SVG QR Codes.

Usage:
```ruby
qrcode.as_svg(:module_size => 6)
```

I also updated the README to hint to the new export parameter - and to hint to the current gem and it's current repo.