`Object.assign` accepts multiple sources

Update up to changes in ECMAScript 6
Rename `toUint` into `toPosInteger`

Technically it's not about unsigned integer, but about any positive
integer representation we can have in JavaScript
Do not limit lists length to UInt32

In ECMAScript 6 objects with length that expand beyond are accepted and
processed as expected
Do not produce sparse arrays in Array.form

As decided by spec: http://esdiscuss.org/topic/array-from-and-sparse-arrays
String.prototype.at

Based on https://github.com/mathiasbynens/String.prototype.at
Rename `String.getFormat` to `String.formatMethod`
-- lint
-- lint
Math.clz32 as implemented in ES6
`isPlainObject(Object.prototype)` should return false
Test for Array.prototype case in isPlainArray
Improve package.json
Remove Makefile (it's cross-env project)
v0.10.0