storage: fix credentials issue.

[stephenplusplus]

Previously, if a user wanted to generate a signed URL, there was no guarantee a credentials object would have already been generated. By itself, generating a signed URL doesn't require a network connection, thus the uncertainty that we would have already fetched a token. This change requires a valid credentials object to exist before returning the credentials object.

This also brings up an issue. We need to go through the code to catch any "maybe async, maybe not" functions, and add in a process.nextTick where necessary. (#166)

[rakyll]

Sorry, I'm not familiar with signed URLs. Is there anyone else to review it @silvolu, from Storage team?

[stephenplusplus]

Np, this is actually mostly an API change - adding the getCredentials method. There weren't actually any functionality changes to the signed URL functionality that we already have.

Currently, we wait until an API request to fetch a token, which makes sense. But to create a signed URL, we don't actually have to make an API call. So, we never get a credentials object (unless the user provided it themselves), which is why I added this method. It will return the credentials if they're available, otherwise it will go build it for us by making a fetchToken call.