Add a heartbeat method to the py.Stream class (more below)

There are a number of ways to implement this, but this is the most
similar to how the rest of the library works. The best approach would
be to return messages from the server along with messages generated
in the library to give the user as much information as possible.
python 3 renames the urlparse module.
Version bump --> 1.4.1 (urlparse name change fix)
Merge pull request #138 from plotly/py3-urlparse-fix

python 3 renames the urlparse module.
Add check for mpl-style datetime data.
Add new test for pandas time series date formatter
refactor mpl_dates_to_datestrings.
Update renderer.py and mpltools.py for change.
update optional requirements
version bump -> 1.4.2 (added support for new date formatter)
Merge pull request #139 from plotly/yet-another-date-formatter

Yet another date formatter
update links
Add an ssl_verification option (more below)

This can be:
- bool (True): always check
- bool (False): never check)
- str: (check certificate at specified path)
Allow users to pass this on sign-in
Insert new verification settings into requests.
version bump --> 1.4.3 (local ssl verification settings)
Add test to check that plotly_ssl_verification setting gets set.
Merge pull request #142 from plotly/set-certificate

Set certificate
Change `print ‘string’` to `print(‘string’)`
Decode byte-strings (more)

1. Plotly uses 2.7 on the backend so it will
return bytes! The json 3.x module expects native
string (unicode), so we need to convert this.

2. base64encode requires bytes, so give it bytes 
and convert after.