storage: support file.compose - fixes #401

[stephenplusplus]

Fixes: #401
Refs: #375

var 2013logs = bucket.file('2013-logs.txt');
var 2014logs = bucket.file('2014-logs.txt');

var allLogs = bucket.file('all-logs.txt');

bucket.combine([
  2013logs,
  2014logs
], allLogs, function(err, newFile) {
  // newFile === allLogs
});

[ryanseys]

This function is very trigger happy when it comes to throwing which is fine I think, in fact, I can think of a couple more scenarios where it may like to throw:

When destination is given as a File but it's for a different bucket. And when sources come from a different bucket then the one being called on... but I'm not 100% sure we need to worry about that, we can always just default to the bucket the method is called on. Just becomes awkward when that file doesn't exist in that bucket or they think they can do cross-bucket composition.

[stephenplusplus]

This function is very trigger happy when it comes to throwing

Only throws in situations where we don't have the minimum required data to let the API call have a chance to succeed.

When destination is given as a File but it's for a different bucket.
And when sources come from a different bucket then the one being called on.

Yeah, I thought of this as well, but avoided as I think the API error should be helpful in that regard. Can implement though if anyone thinks it's important.

[ryanseys]

I agree, delegate to the API here.