Enforce encryption for a notify characteristic

[Stoician]

The current version does not support enforcing encryption when configuring a notify characteristic.
This is something I need for my project and so I added it to get things working.

I think this is a feature that others might like, so attached is a diff of the files that I changed for my project.
Although, this code doesn't distinguish between notify and indicate encryption since there was only 1 bit free in the characteristic flags field.
So you probably can't use the patch directly.

notify_encryption.txt