Feature/extra

[dxinteractive]

Additions

• Added unmutable-extra with pivot function (uniqueBy and averageBy would be good in future)
• Added keyArray() to all collections, which works like keySeq but returns an array of keys.
• Added filter() and filterNot() to unmutable-lite arrays
• Added every(), filter(), filterNot() and some() to unmutable-lite objects.

Fixes

• reduce() no longer unwraps returned data item.
• set() and setIn() now have their values shallowly unwrapped before setting them, to try and prevent accidental nesting of unmutable objects. The same treatment should also go for all methods that accept new values as parameters like unshift() or insert().

[dancoates]

Looks good, just wondering if the nested readme files need to be committed? Just makes the diffs a bit bigger than necessary and they'd still show up on NPM even if they weren't