Fix GitBook links and reference to the repo

[laurent-lao]

Closes #285 Closes #284

I fixed the various paths mentioned in the issues above.

I also changed the various mention to the old repo under https://github.com/nick-thompson/react-juce.git into https://github.com/JoshMarler/react-juce.git. Although these were working fine, it might be confusing for users.