Added license section

Added license section and link to license file.
Fix display of block number in web view; block number is now correctly decoded from the coinbase of a share regardless of its size.
Update Python virtual environment to latest version (fixing an SSL pip vulnerability), and update build instructions to better cache the generated virtual environment.
Update development and testing dependency versions.
Merge pull request #228 from maaku/parse-bip34

Fix display of block number in web view
Merge pull request #229 from maaku/update-virtualenv

Update virtualenv and development/testing dependencies, restructure Makeflie
Merge pull request #217 from benhc123/patch-1

Added license section
Update README.md
Merge pull request #233 from win32re/patch-1

Update README.md
Added --external-ip option

This is the equivalent of the -externalip option of bitcoind.
It allows the public IP address of your node to be explicitly given,
instead of asking the peer to detect it automatically.
This is useful when running from behind dual WAN connections,
assymetric routing, or any other situation in which you want
inbound connections to come in on a different IP address than outbound.
Merge pull request #235 from Krellan/addressfeature

Added --external-ip option
possible fix for #227
Merge https://github.com/forrestv/p2pool into forrestv