`Access-Control-Allow-Methods` & `Access-Control-Allow-Headers` headers
are needed to make POST requests from angular.

The index of the array can be used instead.

Backend improvement (Refactoring)

Content-Type header was not included in response because WriteHeader()
was called before setting Content-Type.

Replace response setup/send code w/ calls to sendResponse()

sendResponse function implements creating and sending a APIRespones so
that this code will not be rewritten for every response situation i.e.
the different http methods.

Now panics when can't connect to database and splits handling of errors
from Open() and Ping() as separate issues.

Moves default case (Method Not Allowed) into else clause so it is not
necessary to return from the previous if's --- only for checking http
method, error checking if statements can/should be returning where
needed.