storage: add gzip option

[stephenplusplus]

Fixes #751
Fixes #752

This adds automatic gzipping to file uploads:

// With upload:
bucket.upload('./input.html', { gzip: true }, function(err, file) {
  // file was gzipped!
  // file.metadata.contentEncoding = 'gzip'
});

// Or with streams:
var file = bucket.file('input.html');
var fs = require('fs');

fs.createReadStream('./input.html')
  .pipe(file.createWriteStream({ gzip: true }))
  .on('complete', function(metadata) {
     // file was gzipped!
     // metadata.contentEncoding = 'gzip'
  });

[callmehiphop]

Looks good to me!