Move the rest of rekor-server over.

Some changes:
- import names (app -> api, logging -> log)
- Commands package (cmd -> app)

Move the Rekor types from server to this repo.

This also breaks the go module dependency on rekor-server.

Create a "cli" directory under cmd.

This is the first step in code reorganization. This follows the patterns
from kubernetes/kubernetes - each binary gets a directory under cmd/.
The main.go file for each binary lives there.
The commands live under cmd//app.

Implement upload command

Accept flags artifact-url,public-key, signature and validate GPG formatting and verify file signature

This change refactors client.go and file.go into an `app` package.

This will make it more easily consumed by rekor-ctl

Once this change is in, I will push rekor-ctl up and then look to
further refactor use of code already duped in rekor-server

Seems to work for Node right now. I haven't tested on linux.

Rename file namespaces to rekord

Simple fix for hardcoded filename

Get Leaf command

Capture case of no previous state file present

Gets a specific leaf by the leaf Index

`rekor getleaf --index ($index)

Based on comment in PR #19

Add an update command to fetch and store the latest rootTreeHash/Size.

If we have a previous set of values, we also check the consistency proof between
these two to make sure the tree has not been tampered with.

General clean up

* Remove unsed files (log_server, inclusion (which is now get)

* Add some meaningful text to CLI help

* Make the README more specific to the new role of the CLI (direct
connections to rekor server)

Proof

Switch the get command over to use the server, and refactor a bit.