Don't bring value of previous calls of handle() to the new one.

Fix #13
Remove self.drop_white_space and make self.outtext into local variable.

I am persuaded that self.drop_white_space is completely dead code, which
never makes any change on anything (self.outtext is in the moment when
it is run always empty string), so its removal doesn’t influence
anything. Therefore, it is possible to make outtext in close() method
into a local variable, which makes whole situation with memory leaks way
more simple.
More PEP8ization and added some docstrings

Also, removed (apparently) useless change of self.outtext on line 381
(self.outtext shouldn't be even initialized at that point)
True/False are built-in constants since 2.3. Really?
Add coverageall to .travis.yml
Merge pull request #12 from mcepl/11_kill_outtext

Don't bring value of previous calls of handle() to the new one.
Fixes https://github.com/Alir3z4/html2text/issues/13