Resolve BeautifulSoup Parser Warning and Unhandled Exception

[seancallaway]

Purpose

This pull request fixes a warning thrown by BeautifulSoup and an error condition that causes the program to crash due to an unhandled exception.

Approach

When ran, the program would display the following error message:

UserWarning: No parser was explicitly specified, so I'm using the best
available HTML parser for this system ("html.parser"). This usually isn't a
problem, but if you run this code on another system, or in a different
virtual environment, it may use a different parser and behave differently.

To get rid of this warning, change this:

 BeautifulSoup([your markup])

to this:

 BeautifulSoup([your markup], "html.parser")

BeautifulSoup recommends using lxml as a faster parser, so I've added lxml to the requirements file and to the BeautifulSoup call.

---

Regarding the unhandled exception, it appears that some items caught by the crawler identify as podcasts, but do no have feedURLs, which causes the program to throw a KeyError exception, which was previously unhandled causing the application to crash.

I've wrapped the JSON loading in a try/except to catch the exception and have the function return None in the case that the JSON doesn't include a feedURL. Of course, this means that the function calling podcast_from_json() needed to be able to deal with None being returned, so I modified that function to only add a Podcast object to the list if it was not None.

[seancallaway]

Also did a bit of cleanup for PEP8.