Update bip-0013.mediawiki

Clarified that the 4-byte checksum is from double SHA256 rounds
Adding link to Go implementation
Merge pull request #8 from WeMeetAgain/patch-1

Adding link to Go implementation of BIP0032
Link to Sarchar's JavaScript BIP32 implementation.
Merge pull request #7 from sarchar/master

Link to Sarchar's JavaScript BIP32 implementation.
Merge pull request #6 from discretepackets/patch-1

Update bip-0013.mediawiki