Make retrieval of server variables pluggable

[atrauzzi]

We're currently using bugsnag on a google app engine application using Laravel.

Unfortunately, the following line is causing us some issues: https://github.com/bugsnag/bugsnag-php/blob/master/src/Bugsnag/Request.php#L62

Now, given that this isn't bugsnag-laravel, I understand that we can't leverage any of the configuration or request/response libraries. Although it would be ideal if the mechanism used to obtain values like these was pluggable. Then, bugsnag-laravel could ship an implementation that retrieves the variables from its own abstraction. Then, bugsnag-php can ship defaults that use the superglobals.

[GrahamCampbell]

Thanks. I definitely plan to get this implemented in v3.

[GrahamCampbell]

Going to implement this using a request resolver to resolve a request object.

[GrahamCampbell]

If there's no request, we can have a nulled object.

[GrahamCampbell]

Will land in 3.x shortly.