Fix detection of 2bit files when uploading them.

When 2bit files are uploaded, they were not detected as 2bit files:

Example:
  test.2bit
    empty
    format: twobit, database: hg19
    The uploaded binary file contains inappropriate content

This issue is solved by registering the TwoBit file format in:
  ./lib/galaxy/datatypes/binary.py

lib/galaxy/datatypes/binary.py

@@ -399,3 +399,5 @@ class TwoBit (Binary):
             return dataset.peek
         except:
             return "Binary TwoBit format nucleotide file (%s)" % (data.nice_size(dataset.get_size()))
+
+Binary.register_sniffable_binary_format("twobit", "twobit", TwoBit)