Fix #17 in both Python 2 and 3

Python 2:
```
(py27-pyfasta-venv)mruffalo@giygas:~/workspace/pyfasta-py2:master$ git rev-parse HEAD
6bc19fc6e04912c6f743e5b203911383ea6fed91
(py27-pyfasta-venv)mruffalo@giygas:~/workspace/pyfasta-py2:master$ git status
# On branch master
nothing to commit, working directory clean
(py27-pyfasta-venv)mruffalo@giygas:~/workspace/pyfasta-py2:master$ python --version
Python 2.7.5+                                                                                               
(py27-pyfasta-venv)mruffalo@giygas:~/workspace/pyfasta-py2:master$ python setup.py nosetests                
running nosetests                                                                                           
running egg_info                                                                                            
creating pyfasta.egg-info                                                                                   
writing pyfasta.egg-info/PKG-INFO                                                                           
writing top-level names to pyfasta.egg-info/top_level.txt                                                   
writing dependency_links to pyfasta.egg-info/dependency_links.txt                                           
writing entry points to pyfasta.egg-info/entry_points.txt                                                   
writing manifest file 'pyfasta.egg-info/SOURCES.txt'                                                        
reading manifest file 'pyfasta.egg-info/SOURCES.txt'                                                        
reading manifest template 'MANIFEST.in'                                                                     
writing manifest file 'pyfasta.egg-info/SOURCES.txt'                                                        
running build_ext                                                                                           
nose.plugins.cover: ERROR: Coverage not available: unable to import coverage module                         
....creating new files:                                                                                     
some.split.fasta                                                                                            
creating new files:                                                                                         
some.0.fasta                                                                                                
some.1.fasta                                                                                                
creating new files:                                                                                         
some.0                                                                                                      
some.1                                                                                                      
creating new files:                                                                                         
some.0.1Kmer.fasta
some.1.1Kmer.fasta
creating new files:
some.0.10Kmer.2Koverlap.fasta
some.1.10Kmer.2Koverlap.fasta
creating new files:
some.split.100Kmer.2Koverlap.fasta
................................................................................................................
----------------------------------------------------------------------
Ran 116 tests in 0.528s

OK
```

Python 3:
```
mruffalo@giygas:~/workspace/pyfasta:master$ git rev-parse HEAD
6bc19fc6e04912c6f743e5b203911383ea6fed91
mruffalo@giygas:~/workspace/pyfasta:master$ git status
# On branch master
nothing to commit, working directory clean
mruffalo@giygas:~/workspace/pyfasta:master$ python --version
Python 3.3.2+
mruffalo@giygas:~/workspace/pyfasta:master$ 2to3-3.3 -w -n . >/dev/null 2>/dev/null
mruffalo@giygas:~/workspace/pyfasta:master$ python setup.py nosetests
running nosetests
running egg_info
creating pyfasta.egg-info
writing entry points to pyfasta.egg-info/entry_points.txt
writing pyfasta.egg-info/PKG-INFO
writing top-level names to pyfasta.egg-info/top_level.txt
writing dependency_links to pyfasta.egg-info/dependency_links.txt
writing manifest file 'pyfasta.egg-info/SOURCES.txt'
reading manifest file 'pyfasta.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
writing manifest file 'pyfasta.egg-info/SOURCES.txt'
running build_ext
nose.plugins.cover: ERROR: Coverage not available: unable to import coverage module
....creating new files:
some.split.fasta
creating new files:
some.0.fasta
some.1.fasta
creating new files:
some.0
some.1
creating new files:
some.0.1Kmer.fasta
some.1.1Kmer.fasta
creating new files:
some.0.10Kmer.2Koverlap.fasta
some.1.10Kmer.2Koverlap.fasta
creating new files:
some.split.100Kmer.2Koverlap.fasta
................................................................................................................
----------------------------------------------------------------------
Ran 116 tests in 0.836s

OK
```