EnableNotifications don't suoport specifying indicate/notify mode

[lemongGH]

EnableNotifications don't suoport specifying indicate/notify mode
In my application，the device service character I used support both indicate and notify, and I need to using indicate mode
But, I found that I can't specifying indicate mode in the method EnableNotifications
And, the code will using the notify mode at default, otherwise the device character not support nofify mode

[lemongGH]

I have fixed this problem and commit a PR.
#293

[jagobagascon]

#293 was merged to dev branch