require project ID

[stephenplusplus]

Fixes #570

// Doesn't throw.
var gcloud = require('gcloud')({ keyFilename: 'key.json' });

// No project-id, will throw:
gcloud.bigquery();
gcloud.datastore.dataset();
gcloud.pubsub();
gcloud.storage();

Other pattern:

// Doesn't throw.
var gcloud = require('gcloud');

// No project-id, will throw:
gcloud.bigquery({ keyFilename: 'key.json' });
gcloud.datastore.dataset({ keyFilename: 'key.json' });
gcloud.pubsub({ keyFilename: 'key.json' });
gcloud.storage({ keyFilename: 'key.json' });

This involved fixing a lot of the examples, but the change is pretty simple.

[jgeewax]

What about

var gcloud = require('gcloud')({projectId: 'myproject'});

Does anything throw now?

[jgeewax]

LGTM

[stephenplusplus]

What about var gcloud = require('gcloud')({projectId: 'myproject'}); Does anything throw now?

No, we only check at the instantiation of the sub-modules, which lets the user wait until the last minute to worry about giving the projectId. So as long as there's a projectId somewhere up the chain or at the time of calling, no errors will be thrown