Begin refactoring feed parsing scripts in podcast

This commit begins refactoring the parse feed code in podcasts to match
the better style of the episode parse_feed code.

It will allow better error catching.

This code works, but it introduces some workaround code relating to
[this stackoverflow](http://stackoverflow.com/questions/22392026/rails-accepts-nested-attributes-for-saves-no-nested-records-if-one-record-is-inv)
question. When that's resolved I'll improve it more.