bug fixes and support for single proxy without removal

[tmendo]

Hi,
I forked your code because I needed to add a single proxy that is not removed upon failure, so I can use pystemon with TOR (which uses a SOCKS proxy, so it needs to be combined with something like delegate that converts HTTP to SOCKS).

In addition I implemented some bug fixes (in bold the ones I consider more important):

• update-min and update-max are now honoured
• added option to enable or disable proxy use
• added option to enable single or random proxy (random proxies are still removed after a few errors)
• disabled redis by default
• redis dependency only loaded if redis is enabled

Since I forked from other fork, this pull also has the following bug fixes:

• prevent dead lock when multiple threads want to remove the same proxy server
• fix encoding problem when sending alert by email
• bugfix for empty textarea on snipt.net
• replaced deprecated method from BeautifulSoup
• added file listener for proxy list and made proxy list a unique set
• bugfix in user agent management

[cvandeplas]

thank you very much.
I believe most of the fixes were already included/merged separately.
I have implemented the proxy feature a bit differently, but it achieves the same result: the system will not remove the last proxy, even if it's failing.