New methods added

New constructors for InstagramConfig addded. Now it is posible to use it
like this:
new InstagramConfig(clientId, secret, redirectUri);
or
new InstagramConfig(clientId, secret, redirectUri, callbackUri);
----------------------
New static method OAuth.AuthLink added. Now it is possible to pass
InstagramConfig instance to AuthLink method and get authentication uri.
Example:
var config =new InstagramConfig(clientId, secret, redirectUri);
var link = OAuth.AuthLink(config, scopes, OAuth.ResponseType.Code);
EnforceSecureHeader refactored

It is unclear that Ips property must be set in addition to
EnforceSecureHeader. This should be single operation (set Ips and
EnforceSecureHeader properties). I've added methods
Enable\DisableEnforceSecureHeader. So now, if user want to use
EnforceSecureHeader he should call EnableEnforceSecureHeader and set
right Ips.
Merge pull request #29 from drussilla/master

New methods for more user friendly use and small EnforceSecureHeader refactoring