CLI JSON object should accept unicode in key

The client can now construct the JSON object sent to the server via CLI using the `--argument key=value` syntax. This commit lifts the unnecessary restriction that the key cannot contain numbers, going further as to allow unicode.

Fixes https://tickets.puppetlabs.com/browse/RAZOR-275
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Show user-friendly error for client/server incompatibility

Currently, razor-client version 0.15.0 cannot run commands with
versions of razor-server prior to 0.15.0, yielding an unexpected
404 from all commands.

So this change is twofold:
1) Supply a nice message when versions are incompatible, and
2) Allow `--json` as a way to bypass this incompatibility.

Fixes https://tickets.puppetlabs.com/browse/RAZOR-278
Version 0.15.1