Non blocking function to transmit

[universam1]

The endPacket() is a blocking transmit that can take over 6 seconds depending on the radio settings.
A non blocking version is proposed with MR #62 , tested successfully on ESP8266.
Note the grace time required for the radio to properly update the registers.

Example added as well to demo.

[ainst]

Would it be possible to add a callback as well as part of the changes to endPacketAsync()?

For example, I would like to turn receive back on once a transmit has been completed.

[torntrousers]

Agree with @ainst , a callback for tx done would make this async support much more useful.

[sandeepmistry]

Closed via #62.

Would it be possible to add a callback as well as part of the changes to endPacketAsync()?

@ainst @torntrousers please open a new pull request for this.