Now able to override HTTP client

Adding a package variable so that the HTTP.client can be overridden. This would be needed if used on Google App Engine.

Example code
```go
yubigo.HTTPClient = urlfetch.Client(c)  // c is your context
yubiAuth, err := yubigo.NewYubiAuth(YUBIKEY_CLIENT_ID, YUBIKEY_SECRET_KEY)
```