Add support for basic digital ocean spaces API

[rpodcast]

Description

This PR brings preliminary support for utilizing the digital ocean spaces object storage API. The spaces API is backward-compatible with the Amazon S3 object storage API, so I leverage key functions from the aws.s3 package to work with the API. Since the authentication is controlled by separate keys from the normal digital ocean API, the user is expected to create these keys within their DO spaces account, and the user can pass the key ID and secret directly in the functions or define them as environment variables (DO_SPACES_ACCESS_KEY and DO_SPACES_SECRET_KEY). The main functions introduced are the following:

• spaces() : Obtain all spaces in your account. Each one will be given a class called space with custom print and summary S3 methods.
• spaces_GET() : Supporting function that wraps aes.se::s3HTTP() to obtain the spaces information using the API.
• space_info() : Obtain metadata around all files contained in a space. Note that if the space contains more than 1,000 files that this operation can take a bit of time, since the S3 API only allows for up to 1,000 files to be queried at a time.

Related Issue

136

Example

# obtain the spaces in your DO account
my_spaces <- spaces()

# print summary information of the space called "test-space"
summary(my_spaces[["rpodcast-space3"]])

<space_detail>rpodcast-space3
  Size (GB):     37.54697 
  Files:        17331
  Created at:    2017-08-27T19:01:05.737Z 

# create a new space in your account
space_create("my-new-space")

[rpodcast]

Thanks for your review @sckott! I've added comments above and pushed the updates.

[sckott]

• It's not a great idea to use internal variables that are the same name as the function they are within, e.g,. spaces within spaces function

[rpodcast]

I finally got a chance to make updates and address your latest comments. Please take a look and let me know if you see other areas that need revisions.

[sckott]

Looks good @thercast

Let's merge this, then we can go back to #136 and discuss how to divy up tasks