Fix bug where correct() stripped punctuation

See #48
On branch dev
Your branch is up-to-date with 'origin/dev'.

Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

	modified:   HISTORY.rst
	modified:   tests/test_blob.py
	modified:   textblob/__init__.py
	modified:   textblob/blob.py