Add SqlCredential support -

Add SqlCredential support -
- Tds.cs: Use SecureString objects for passwords; Add method to retrieve string from SecureString
- Tds42.cs: Use SecureString for passwords
- Tds50.cs: Use SecureString for passwords
- Tds70.cs: Use SecureString for passwords
- TdsConnectionParameters.cs: Use SecureString for passwords; Initialize Password parameter as cleared string; Add indicator for when password is set
- SqlConnection.cs: Use SecureString for passwords; Add SqlConnect method that accepts an SqlCredential along with the Connection string; Perform checking of parameters to ensure user/password not specified in connection string if credentials have been specified or using credentials when domain login is specified.
- SqlCredential.cs: Add new class with support for credentials
- System.Data.dll.sources: Add SqlCredential.cs to the build list