Added rudimentary regular expression support

for matching incoming messages against a specified RegEx filter.
Show more useful HTTP and server error messages

- display HTTP status code when not "200 OK"
- display custom server "error" messages, if any.

Significant code structure changes:
- renamed extractPayloadJSON() to getJsonSuccessStatus()
- overloaded showToast() to handle strings
Add guidance for creating first Sync URL.
Merge pull request #79 from olliebennett/develop

User-Friendly errors and RegEx support