LoginDialog Password Preview Feature

Adding the ability to use the Password Preview feature like in Windows 8 login to the LoginDialog.  It's optional and doesn't have to be used.  I used the eye icon in the MahApps icon library for the password preview button.

I merged templates and styles from MahApps & from WPFSmartLibrary.  I integrated in what was needed from the WPFSmartLibrary into the MahApps w/o adding a DLL reference to WPFSmartLibrary (https://wpfsmartlibrary.codeplex.com/).

![mahappslogindialogpassprevmenuitem](https://cloud.githubusercontent.com/assets/1051590/4238377/dab049ac-39d7-11e4-80f6-bf2facc4eed6.png)

![mahappslogindialogpassprevinitscreen](https://cloud.githubusercontent.com/assets/1051590/4238380/e209f572-39d7-11e4-9641-10eb635067e8.png)

![mahappslogindialogpassprevpasswordtyped](https://cloud.githubusercontent.com/assets/1051590/4238384/ec80a0c8-39d7-11e4-84aa-2580cd559542.png)

![mahappslogindialogpassprevhoveroverpreviewbutton](https://cloud.githubusercontent.com/assets/1051590/4238388/f4337de0-39d7-11e4-995b-a3717cb5014c.png)

![mahappslogindialogpassprevclickedonpreviewbutton](https://cloud.githubusercontent.com/assets/1051590/4238393/ffaf066c-39d7-11e4-9613-1fce62e830c8.png)

![mahappslogindialogpassprevpreviewbuttonmouseup](https://cloud.githubusercontent.com/assets/1051590/4238396/02dcb38e-39d8-11e4-988c-3b5839fc9167.png)