Not re/using default HTTP Client

[MarinX]

When doing a request, each new client is created with none default values.
This can cause a problem if something happens with API and client can hang infinitive because there is no timeout set.

Also, as suggested by go doc: Clients should be reused instead of created as needed. Clients are safe for concurrent use by multiple goroutines.
It should also let the users set their own HTTP client, if none, use http.DefaultClient

If this is something you agree on changing, I can create a valid PR for it.