External provisioning of a document store

Hey @chrish619 @chrisaswain,

I spend some minutes and spiked out a way to provide an `IDocumentStore` instead of only relying on the connection string.

First step is to modify your elmah settings in the web.config

    <elmah>
     <errorLog type="Elmah.RavenDbErrorLog, Elmah.RavenDB" UseExternalStore="True" connectionStringName="Elmah" />
    </elmah>

There is a new config key named `UseExternalStore`. If it is set to true, you _have_ to provide an instance of a DocumentStore. The connection string will be ignored.

At the start of your application create a DocumentStore e.g. a `EmbeddableDocumentStore` and configure it. 

Then you could pass this instance to the RavenDbErrorLog

    RavenDbErrorLog.ConfigureWith(_store);

After this it will use the provided instance.

Thoughts?

/cc @GregBrant