Sites

With the growing number of binstar sites available, it is becoming harder to manage the sites with the existing config system. This pull request adds a `-s/--site` option to the `binstar` and `binstar-build` commands.

This allows quick or temporary switching between sites. The previous approach to setting a url was:

    binstar config --set url http://localhost:4990/api

This approach will still work but now it is preferred to define a new site:

    binstar config --set sites.sitename.url http://localhost:4990/api

The command:

    binstar -s sitename login

Will log in to the http://localhost:4990/api site url

To omit the `-s sitename` argument, you can set a default site:

    binstar config --set default_site sitename

Then the simple command:

    binstar login

Will log in to the http://localhost:4990/api site url

### Note:

    binstar -s binstar whoami
    binstar -s alpha whoami

Are included by default and will now **always** point to http://binstar.org and http://alpha.binstar.org respectively.