Other currency sources

[racitup]

Hi, I've completely refactored the management commands (maintaining backward compatibility of course) to add in some new features and Yahoo Finance integration. New features and advantages:

• An alternative to being tied into openexchangerates
• YF provides symbols so you don't have to keep a file up-to-date
• Added ability to get the list of currencies from settings
• Added ability to get the desired base rate flag from settings
• Automatic DB base rate change (even with a free oxr account)
• Simple to add Google Calculator, ECB, etc handlers
• I'll also update the documentation..

Downsides:

• I've had to scrape the YF content and I guess there's no guarantee that things will stay where they are now
• I've only tested on Django 1.9.9, BeautifulSoup 4.4.0, requests 2.11.1, urllib3 1.12, openexchangerates 0.1.1 and python 3.5.2
• I haven't written any test cases

Happy to take feedback if there are any major headaches in there but really need to get back to the 'day job' ;) Would love to see this merged!

Check it out!

[panosl]

It would be great if we could have some form of testing though, feel free to make another pull request so we can have this fixed.