fix #23: separate modules for endpoints and framework

[adamchalmers]

I've reorganized the root of the repo. Previously there were two kinds of root modules: modules for particular Cloudflare endpoints (e.g. DNS or WorkersKV) and modules for the fundamental API framework (e.g. response, apiclient, auth).

This PR restructures the root of the crate so there are only two root modules: framework and endpoints.

Before:

src/
    account.rs
    apiclient.rs
    auth.rs
    dns.rs
    lib.rs
    mock.rs
    plan.rs
    response/
    workerskv/
    zone.rs

After:

src/
    endpoints/
        account.rs
        dns.rs
        plan.rs
        workerskv/
        zone.rs
    framework/
        apiclient.rs
        auth.rs
        mod.rs
        mock.rs
        response/

This will make it much more clear for contributors where their code should go. Are you adding a new endpoint? endpoints module. Are you changing the underlying API framework? framework module.

I've already integrated these changes into a branch of tunnelmon, and it was very straightforward.

[sssilver]

@ashleymichal has a PR up that adds even more taxonomy to this.

[ashleymichal]

i would like to include a CONTRIBUTING.md or similar like that introduced in #2 ; that could be a subsequent PR though.

[adamchalmers]

Added a CONTRIBUTING.md!