Implement create/update and list Workers endpoints

[ispivey]

This builds on changes in #105 to implement the simple version of the create/update Worker endpoint, as well as implementing the list Workers endpoint, as described in #104 . Up next can be the multi-part-form create/update method!

... I'm not sure the best way to stage my PRs so that this one clearly depends on my previous one, since I realize each PR should correspond to a single issue. I'd appreciate some guidance :)

To validate this all works as intended:

➜  cloudflare-rs git:(ispivey/workers-list-and-create-scripts) cd cloudflare-examples
➜  cloudflare-examples git:(ispivey/workers-list-and-create-scripts) cargo run -- --auth-token=$CF_AUTH_TOKEN create_script $CF_ACCOUNT_ID bleepbloop | jq
    Finished dev [unoptimized + debuginfo] target(s) in 0.14s
...
{
  "id": "bleepbloop",
  "etag": "005603661bf9ed84cb3569a53f7caf7c3377e726b37e6064001afb2462ae9845",
  "script": "addEventListener('fetch', event => { event.respondWith(fetch(event.request)) })",
  "size": 127,
  "created_on": "0001-01-01T00:00:00Z",
  "modified_on": "2020-02-09T18:26:34.653254Z"
}
➜  cloudflare-examples git:(ispivey/workers-list-and-create-scripts) cargo run -- --auth-token=$CF_AUTH_TOKEN list_scripts $CF_ACCOUNT_ID | jq
   Compiling cloudflare v0.6.3 (/Users/ispivey/src/cloudflare-rs/cloudflare)
   Compiling cloudflare-examples v0.5.0 (/Users/ispivey/src/cloudflare-rs/cloudflare-examples)
    Finished dev [unoptimized + debuginfo] target(s) in 7.80s
...
[
  {
    "id": "bleepbloop",
    "etag": "005603661bf9ed84cb3569a53f7caf7c3377e726b37e6064001afb2462ae9845",
    "script": null,
    "size": null,
    "created_on": "2020-02-09T02:29:07.292152Z",
    "modified_on": "2020-02-09T17:30:24.938709Z"
  },
...
]
➜  cloudflare-examples git:(ispivey/workers-list-and-create-scripts)