Make keyring bundable

Not sure if there is some interest there: we would like to make keyring 'bundlable' inside one of our project. Given the nature of keyring, I think other projects (e.g. pip, etc...) may be interested in bundling keyring instead of depending on it as a dependency.

To make that possible, we converted absolute imports to relative imports. The test suite passes on travis-ci and on my mac.