Implement send invoke

[slipo]

This was originally done here:
https://github.com/CityOfZion/NeoLink/blob/master/src/js/background.js#L344

Using a custom fork of neon-js:
uvmetal/neon-js@18c1ec4

We want to reimplement this in the React version of the app in the development branch. We want to continue using the CityOfZion neon-js branch so research needs to be done into whether it's currently possible to do this in neon-js v2 or if we can work with them to add it.

[slipo]

I'll take this one on. @snowypowers has given me tips on how to accomplish this.