Keyring moved to Github for hosting

[Jason R. Coombs]

Given the dominant preference for Git and the simple integration tests on Travis-CI, I've moved the Keyring library from Bitbucket (/kang/python-keyring-lib) to Github (/jaraco/keyring) for hosting.

I've made a separate release (5.1) for this change, so even though the hosting doesn't change, the readme has been updated to reflect the new hosting.

I've migrated the issues and source history, so there should be no reason to keep the Bitbucket project, though I will leave it active for now.

If you had access to the repository on Bitbucket and would like the same access on Github, please don't hesitate to let me know and I'll make that happen ASAP.