Update Decryption class so that it matches specification

[ghost]

Python 3.6.5 (default, Apr 14 2018, 13:17:30) 
[GCC 7.3.1 20180406] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from lantern.modules import shift
>>> from lantern import fitness
>>> ciphertext = "iodj{EuxwhIrufhLvEhvwIrufh}"
>>> decryptions = shift.crack(ciphertext, fitness.english.quadgrams)
>>> print(decryptions[0])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __str__ returned non-string (type list)

I think it makes sense to have the plaintext stored as a list. If it's to be kept as a list, the __str__ method should be updated to join the list prior to returning the plaintext. The documentation would need to be updated accordingly as well. For now, I've updated the Decryption class so that self.plaintext is a string as it meets the spec.

[coveralls]

Coverage remained the same at 99.569% when pulling e512319 on jtalowell:master into 235e163 on CameronLonsdale:master.

[ghost]

This is on the current master branch.

Python 3.6.5 (default, Apr 14 2018, 13:17:30) 
[GCC 7.3.1 20180406] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from lantern.modules import shift
>>> from lantern import fitness
>>> ciphertext = "iodj{EuxwhIrufhLvEhvwIrufh}"
>>> decryptions = shift.crack(ciphertext, fitness.english.quadgrams)
>>> print(decryptions[0])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __str__ returned non-string (type list)
>>> from lantern.modules import simplesubstitution
>>> ciphertext = "iodj{EuxwhIrufhLvEhvwIrufh}"
>>> decryptions = simplesubstitution.crack(ciphertext, fitness.english.quadgrams)
>>> print(decryptions[0])
oldi{NtureOftheSaNearOfthe}

The inconsistency here probably needs a better fix than just joining the string (this pull request). The Decryption class should either accept a string or a list and the other modules should be changed accordingly.

[CameronLonsdale]

Thanks for this PR, but for now the str representation will be removed in favor of manually joining decryptions