Fixed Py3 compatibility

[frenzymadness]

Hello.

I downloaded the latest release of the mailer from PyPI and when I tried to run python setup.py install with Python 3.5.1 I get the following error:

byte-compiling build/bdist.linux-x86_64/egg/marrow/mailer/transport/postmark.py to postmark.cpython-35.pyc
  File "build/bdist.linux-x86_64/egg/marrow/mailer/transport/postmark.py", line 68
    except (urllib2.HTTPError, urllib2.URLError), e:
                                                ^
SyntaxError: invalid syntax

It looks like Python 3 incompatibility so I've changed this line and now installation works for Python 2 and 3.

[frenzymadness]

I have one more question. Could you please make a new release on PyPI with README.textile and LICENSE.txt included in tarball?

Thank you.

[codecov-io]

Current coverage is 85.39%

Merging #67 into develop will not change coverage

@@            develop        #67   diff @@
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  Files            10         10          
  Lines           767        767          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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  Hits            655        655          
  Misses          112        112          
  Partials          0          0          

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