Added SDCTheme protocol to change the colours and fonts of the alert view

Added a possibility to theme the SDCAlertView

````
SDCAlertView *alert = [[SDCAlertView alloc] initWithTitle:@"Title"
															  message:@"This is a message"
															 delegate:nil
													cancelButtonTitle:nil
													otherButtonTitles:@"OK", nil];
alert.theme = [[SDCTestTheme alloc] init]; // implement the SDCTheme protocol methods you want to change

[alert show];
````