Added a #merge method to the map classes.

[jdantonio]

Addresses #8.

The new '#merge` method for the map classes has the same behavior as the same method on a Ruby hash, including the ability to pass a block. The return value is a new map with the appropriate values updated accordingly:

m = Ref::WeakKeyMap.new #=> {}
a = Object.new #=> #<Object:0x007fe072421ee0>
m[a] = "test" #=> "test"
n = m.merge(Object.new => "something") #=> {#<Object:0x007fe072421ee0>=>"test", #<Object:0x007fe072409250>=>"something"}