Fix for the name change bug

[NoahRosa]

This PR should fix #6 with two limitations:

1. The device name can't be more than 7 characters long. If the string passed to the begin function is longer, it is automatically trimmed to 7 characters.
2. The android application designed to work with the SensorDemo example expects the name BlueNRG. Otherwise the device can't be opened.

[cparata]

@fpistm , what about this PR? Could we integrate it in the master repository?

[fpistm]

Additionnal comment, maybe the restriction could be added in the README.md and/or sketch header?

[NoahRosa]

I modified the commit in order to include the changes you requested.
I added the limitation description both in the README.md and the sketch header.
Sorry about the comment, I added it as the last thing and forgot to recompile.

[fpistm]

No worries Noah. This also happen to me. 😉