Bug 1518759 - register APIHandler against /api/ rather than /api

[petemoore]

See bug 1518759 for details.

Fallout from #38.

The ServeMux docs state:

ServeMux is an HTTP request multiplexer. It matches the URL of each incoming request against a list of registered patterns and calls the handler for the pattern that most closely matches the URL.

Patterns name fixed, rooted paths, like "/favicon.ico", or rooted subtrees, like "/images/" (note the trailing slash).

Thus, we need to register the APIHandler against /api/ rather than /api (which would only match the fixed url path /api).

[petemoore]

CC @djmitche @walac @grenade