Sets & maps append-multi. Also docs and a fix for SortedSets

[laher]

Finishing touches for #32 . Mainly docs and appending multiple items at once.

Set/SortedSet

• varargs (as with recent PR for Lists)
• docs: readme and godoc
• SortedSet only: rename Put() to Set(), in line with the other types' methods. (I originally had both Set.Put(...) too, but forgot to update SortedSet in line with Set)
• Items() returns a slice of the set contents.

Map/SortedMap

This is similar to the Varargs for sets / lists. You can't really do varargs in the same way (unless you added an Entry type, but it would be clunky IMO), so I just added separarate funcs which take a map[K]V instead.

• NewMapOf(Hasher, map[K]v) / NewSortedMapOf(Comparer, map[K]v)
• SetMany(map[K]v)