dev-util/electron: Add ebuild for Github's Electron (v0.36.9)

dev-util/electron: Add ebuild for Github's Electron (v0.36.9)

Electron is an application framework built on top of Chromium and NodeJS.
It allows creating cross-platform desktop apps using Web tech. [1]

The main user of Electron is the Atom IDE [2].  Microsoft Visual Studio
Code is also based on Electron.  It is also useful for the creation of
desktop wrappers for popular web services, e.g. Google Music [3].

Unfortunately, current upstream's approach to "building Electron from
source" involves downloading a pre-built libchromiumcontent.so binary
from the Internet, which poses obvious security and compatibility risks.

This ebuild is based heavily on the Chromium ebuild and follows similar
unbunling policy.

Additionally, Electron-specific NodeJS headers are installed to
/usr/include so that native modules built for Electron-based apps can be
compiled against the correct version of NodeJS embedded in Electron.

[1] http://electron.atom.io/
[2] http://atom.io/
[3] https://github.com/twolfson/google-music-electron

Gentoo-Bug: https://bugs.gentoo.org/579116
Closes: https://github.com/gentoo/gentoo/pull/1200
Package-Manager: portage-2.2.28

Acked-by: Mike Gilbert <floppym@gentoo.org>
Signed-off-by: Patrice Clement <monsieurp@gentoo.org>