Add extra field in base.Content

This field should be used to add information that was gathered in the process.
Add periscope and BeautifulSoup to dependencies

Periscope is the library we use to handle subtitle search for now, it requires
BeautifulSoup to be installed.

There is a glitch right now when installing in develop mode. Periscope requires
BeautifulSoup but from some reason it installation fails forcing me to install
BeautifulSoup before calling setup.py. Will work on that later.
Subtitle search through periscope

Work in progres.
Add support to search for subtitiles using periscope.