Add file_to_protein method, rna.txt file, test for file_to_protein
method

In lib/Bio/TranslateRNA.pm
  Added file_to_protein method which takes a file containing an RNA
  string and converts it to protein

In t/rna.txt
  Added valid RNA string

In t/translate_rna.t
  Added subtest:
    Test file_to_protein method with valid RNA string