WIP : Added full proxy support

[gpailler]

This PR introduces full proxy support for SlackClient, SlackTaskClient and SlackSocketClient and supersede #133.

I had to do some changes and API breaks to be able to pass a IWebProxy instance in SlackClient/SlackTaskClient and SlackSocketClient constructors. Most of the changes are only code move. I added comments on real changes

• SlackClientBase is created and contains generic web access methods (HttpWebRequest/HttpClient with proxy support)
• SlackClient and SlackTaskClient inherit SlackClientBase
• SlackSocket is updated with proxy support
• [API BREAK] SlackSocketHelpers class is created and contains the old public static methods from SlackClient converted to instance methods
  • void StartAuth(Action callback, string email)
  • Task StartAuthAsync(string email)
  • void FindTeam(Action callback, string team)
  • Task FindTeam(string team)
  • void AuthSignin(Action callback, string userId, string teamId, string password)
  • Task AuthSignin(string userId, string teamId, string password)
  • Uri GetAuthorizeUri(string clientId, SlackScope scopes, string redirectUri = null, string state = null, string team = null)
  • void GetAccessToken(Action callback, string clientId, string clientSecret, string redirectUri, string code)

Because of these changes, I think the next SlackAPI version should be labelled 1.1.0