Added the universal 'respond_to("method")' and 'is_a(Object)' methods.

Example:
	"string".respond_to('uc');  # true
	"string"is_a(String);       # true
        {2+3}.is_a({});             # true
	[1,2].is_a(Array);          # true
        [1,2].is_a("Array");        # false

Added the 'downto' method for strings.

Example:
	"c".downto("a");            # returns: ["c", "b", "a"]