support Tor about spruned

[gdassori]

--enable-tor
--force-tor to ensure spruned will connect only to electrum hidden services.

[gdassori]

Design review.
There are two ways to use Tor:

--tor

• if no proxy is specified with the --proxy command, the default 'localhost:9050' is used
• this mode establish connections with hidden services only

--proxy localhost:9050

• passing the tor proxy (i.e. localhost:9050) without using the --tor flag will enforce its usage, but not the hidden services policy: spruned would instead use tor exit nodes to connect to clearnet peers
• only socks5 are supported

--proxy and --tor can be used together, and would enforce both the hidden services policy and the usage of the specified proxy.

[gdassori]

PR #76 merged, Tor is supported is in master.
No Pypi release yet
(See https://stackoverflow.com/questions/20101834/pip-install-from-git-repo-branch if you want to use pip)