storage: add getSignedPolicy function to generate a signed policy

[mziccard]

Added a function to generate a signed policy for POST requests.
So far it supports several conditions (according to the docs):

• Equality: ["eq", "$<field>", "<value>"]
• Starts with: ["starts-with", "$<field>", "<value>"]
• Content length: ["content-length-range", <min_range>, <max_range>]
• Bucket: {"bucket": "<bucket_name>"}
• Acl: {"acl": "<acl_name>"}
• Success redirect: {"success_action_redirect": "<url>"}
• Success status: {"success_action_status": "<status>"}

Documentation is provided describing function arguments.
Unit tests are added.

[stephenplusplus]

All looks good to me. Thanks for adding this @mziccard!

@ryanseys anything left to resolve before merge? I'll handle the small style issues, so we don't bug him with that :)

[mziccard]

@stephenplusplus Feel free to bug me:)

BTW, I think it is still open for discussion whether to move this method to Bucket (allowing signed policies not associated to a specific filename) or not.

Adding a regression test might be useful as well.

[mziccard]

Should be ok now, even the boring stuff :)
Both

file.getSignedPolicy({equals: ['$Content-Type','image/jpeg']// ...

and

file.getSignedPolicy({equals: [['$Content-Type','image/jpeg']]// ...

are now supported if only one condition is specified. (also for startsWith condition).

[stephenplusplus]

Thank you very much!

[mziccard]

Everything ok on my side, are there any more thoughts?

[ryanseys]

Just a couple console.logs left in the tests, but otherwise looks ready for merge! :)