Use native Object.assign instead of xtend

[jimmywarting]

xtend can easily be replaced with Object.assign

This reduce the dependencies and possible also avoid duplicated versions where some package don't use the same version range for the xtend package

// immutable
Object.assign({}, a, b)

// mutable
Object.assign(a, b)

[goto-bus-stop]

I appreciate the commitment but please don't open issues for every browserify package that uses xtend. This module is part of browserify so its appreach will be the same as the main browserify package. Until there is a particularly good reason to drop support for Node < 4 it won't happen. It takes no maintenance effort to use xtend so I really don't see a reason to move away.