When parsing interval 1min is interpreted as 1second rather than quitting with a parse error

It would be nice if graphite-beacon quit with a parse error like the JSON parse error when it encountered time units it didn't understand.

When I pass in:
"interval": "1min",
by mistake instead of "1minute" it will actually poll every second.

Start by locating the configuration interval parser and the handling for JSON parse errors. Add coverage for the invalid value "1min" and verify that graphite-beacon exits with a parse error instead of treating it as one second.

Written by the indexing model from the issue text.