improved functionality of repr
improved functionality of repr

Before:
======================================================================
FAIL: complement functions as expected
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tests/test_core/test_sequence.py", line 183, in test_complement
    self.assertEqual(self.b1.complement(),DNASequence("CTAATGT"))
AssertionError: CTAATGT != CTAATGT

After:
======================================================================
FAIL: complement functions as expected
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tests/test_core/test_sequence.py", line 183, in test_complement
    self.assertEqual(self.b1.complement(),DNASequence("CTAATGT"))
AssertionError: <NucleotideSequence: CTAATGT (length: 7)> != <DNASequence:
CTAATGT (length: 7)>

anderspitman/scikit-bio,johnchase/scikit-bio,Kleptobismol/scikit-bio,Kleptobismol/scikit-bio,jensreeder/scikit-bio,colinbrislawn/scikit-bio,kdmurray91/scikit-bio,demis001/scikit-bio,xguse/scikit-bio,Jorge-C/bipy,jdrudolph/scikit-bio,anderspitman/scikit-bio,colinbrislawn/scikit-bio,averagehat/scikit-bio,corburn/scikit-bio,SamStudio8/scikit-bio,xguse/scikit-bio,averagehat/scikit-bio,Kleptobismol/scikit-bio,gregcaporaso/scikit-bio,jairideout/scikit-bio,johnchase/scikit-bio,kdmurray91/scikit-bio,jensreeder/scikit-bio,Achuth17/scikit-bio,wdwvt1/scikit-bio,SamStudio8/scikit-bio,Achuth17/scikit-bio,corburn/scikit-bio,gregcaporaso/scikit-bio,demis001/scikit-bio,jairideout/scikit-bio,jdrudolph/scikit-bio,wdwvt1/scikit-bio