A more complete C example

[jonwoodring]

A more complete C example:

• adds the threadsafe cleanup code
• signal handlers for SIGINT and SIGTERM (same exit code)
• modified README for a bit more detail
• added a simple level logger
• adds a Makefile and two ways to make through pkg-config and without
• changed the environment variables to DELTACHAT_C_ECHOBOT_EMAIL and DELTACHAT_C_ECHOBOT_PASSWORD
• changed if-else chains to switch-case

[adbenitez]

Hi @jonwoodring thanks a lot for contributing!!! I feel like this is changing too much things in a single pull request but I am no C developer, maybe our C expert @r10s can review your PR

[jonwoodring]

That's fine.

If it's too much, I can just keep it alive on my page, and maybe put it link in the README or so someone else can find it. Or maybe put is as an alternate example in a second directory?

Either way, no skin off my back. Thanks.