More collection functions, CollectionTrait and bugfixes

There are some new functions, specifically:

• second() // seems useless but really usefull :)
• combine($values) // uses current collection as keys, $values as values
• except($keys) // rejects every item with key found in $keys
• only($keys) // filters only items with key found in $keys
• difference(...$collections) // filters items that are in the original collection but not in any of $collections
• flip() // flips keys with values
• has($key) // checks for the existence of item with $key key

CollectionTrait has been introduced and its usage documented in readme.

A handful of bugfixes also:

• Collection constructor might have experienced conflicts between callable (in array form) and array arguments
• Pluck might have failed on heterogenous collections. Now ignores non-collection items.