Use http.DefaultClient with a timeout

[jarifibrahim]

The http.DefaultClient doesn't have a timeout. See https://golang.google.cn/src/net/http/client.go .
No timeout means the client would wait indefinitely for the response. We shouldn't wait indefinitely for a response.
I've added a 15 seconds timeout which should be more than enough for any request.

[kwk]

A timeout is good but this only adds a one-time timeout. How about retrying for n number of times?

[corinnekrych]

we can improve the timeout mgt later, have it configurable... but better get one than none. +1

[jarifibrahim]

A timeout is good but this only adds a one-time timeout. How about retrying for n number of times?

I've created #32 to track this.

we can improve the timeout mgt later, have it configurable... but better get one than none. +1

I've created #33 to track this.