storage: add file#save

[stephenplusplus]

Fixes #1179

This adds file#save, which is a simple wrapper around file#createWriteStream:

file.save(new Buffer('buffer'), function(err) {});
file.save('string', function(err) {});

[coveralls]

Coverage decreased (-76.9%) to 23.146% when pulling 83f95b0 on stephenplusplus:spp--1179 into 8d38290 on GoogleCloudPlatform:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 495bc0a on stephenplusplus:spp--1179 into 8d38290 on GoogleCloudPlatform:master.

[callmehiphop]

@stephenplusplus I realize this is just a convenience method, but what are your feelings on adding a system-test?

[stephenplusplus]

Added!

[coveralls]

Coverage remained the same at 100.0% when pulling 92a7e23 on stephenplusplus:spp--1179 into 8d38290 on GoogleCloudPlatform:master.

[asciimike]

Thanks for doing this!

[stephenplusplus]

Thank you for the idea :)