Add a new error type, TwoFactorAuthError, which identifies the need to
include a one-time password when using Basic Auth for a user with
two-factor auth enabled.

An example of using both the new transport and error type can be seen in
examples/basicauth/main.go.

fixes #258

The RateLimit() func is deprecated, and should not appear in the
official example.

notably, this also updates client.Do() to handle non-JSON response
bodies by passing an io.Writer as the final argument, v.

This provides somewhat reasonable string representations of GitHub
structs.  This is specifically designed for this library and takes a
number of shortcuts where it can, so is not suitable as a general
purpose solution.  That said, I am exporting this function because it is
useful for things like printing out slices of GitHub structs, as can be
seen in examples/example.go.

I am certainly open to suggestions for what exactly the stringified
output should look like.  Currently, I think I've found a reasonable
compromise between fmt's "%v" and "%s" output.

Since the library has received external contributions, Google is no
longer the sole copyright holder.  Update the boilerplate at the top of
each file to reflect that, and add AUTHORS and CONTRIBUTORS files to
track this information.

This now aligns with the (pretty strong) golang convention of the
package name matching the path basename

This commit adds the core library funcationality and establishes the
general calling style and testing structure.  Only a few GitHub API
methods related to organizations and repositories are included in this
first commit, mainly to cement the calling style.