Use electron builder NSIS installer

[evertonfraga]

Currently, our build process takes care of building NSIS (windows installer) on its own. We should migrate it to electron-builder way, to simplify our process and take advantage of electron-builder code signing functionality (#3444).

• add NSIS target to the list
• port this nsi script to the new solution
• remove build-nsis from gulp tasks.

Where to begin:
Looking at gulpFile.js and w

Related code:

• https://github.com/ethereum/mist/blob/develop/gulpfile.js#L85
• https://github.com/ethereum/mist/blob/develop/gulpTasks/building.js#L282-L295
• https://github.com/ethereum/mist/blob/develop/gulpTasks/building.js#L144
• https://github.com/ethereum/mist/blob/develop/scripts/windows-installer.nsi