Added Indications Support for Linux implementation

[demon-xxi]

Please review my changes and consider merging into your repo.

Added new method Peripheral.SetIndicateValue to subscribe for gatt indications similar to notifications. For Indications though we need to send acknowledgment which is done by

p.l2c.Write([]byte{attOpHandleCnf})

Current pr contains only linux implementation. Darwin version needs more investigation as seems less documented and uses some hard-coded values instead of constants.

[roylee17]

Looks good to me for the linux implementation.
To make it an API available for users, you might want to:

• Add it to the interface in peripheral.go, Nice doc comment (with notes of the difference/status between Linux and Darwin).
• Add a place holder (a working version, even better) for Darwin version in peripheral_darwin.go.

Thanks a lot.

Roy

[demon-xxi]

Good catch Roy, I will do updates today.

[demon-xxi]

I've added method to interface and a placeholder for darwin.
Also crated issue #32 to address that and the note in readme.md file. Will definitely get back to tat one when have some spare time. Seems easy to do but need to test carefully.

[roylee17]

Merged, thanks!