Add a noConflict function

The noConflict function returns the Bugsnag object and restores the
previous value assigned to window.Bugsnag.

This allows javascript developers to use Bugsnag in their code without
affecting window bindings at all - essential for bookmarklets and other
3rd party embedded code.

This way, an existing Bugsnag setup won't be conflicted with a 3rd party
javascript file (using a different API key). It is assumed that when
using the noConflict function, developers would likewise ensure that
`autoNotify` is set to `false` to prevent spurious exceptions from
outside their code being reported to their Bugsnag account.
Document the noConflict function
Merge pull request #22 from altmetric/noConflict

Add a noConflict function