Solve in Python:
In genetic the reverse complement of a sequence is formed by **reversing** the sequence and then taking the complement of each symbol.

The four nucleotides in DNA is Adenine (A), Cytosine (C), Guanine (G) and Thymine (Thymine). 

- A is the complement of T 
- C is the complement of G.

This is a bi-directional relation so:

- T is the complement of A
- G is the complement of C.

For this kata you need to complete the reverse complement function that take a DNA string and return the reverse complement string.

**Note**: You need to take care of lower and upper case. And if a sequence conatains some invalid characters you need to return "Invalid sequence".

This kata is based on the following [one](http://www.codewars.com/kata/complementary-dna/ruby) but with a little step in addition.
def reverse_complement(dna):
    table = str.maketrans("ACGT", "TGCA")
    return "Invalid sequence" if set(dna) - set("ACGT") else dna.translate(table)[::-1]