New feature - Alternate Text for sharers

[zysoft]

I've added new feature - alternate text for sharers. It works globally and doesn't require sharers code change, so it's transparent. It allows you setting custom text which will be shared if the give sharer is selected. Sharers are identified by their title (exactly as people see them in UIActionSheet which should be clear for users).
Examples updated.

Example right here:

    SHKItem *item = [SHKItem text:@"This is a text to share via ShareKit"];
    [item setAlternateText:@"This is a text to share via #ShareKit" toShareOn:@"Twitter"];
    [item setAlternateText:@"This is a text to share via Email" toShareOn:@"Email"];
    SHKActionSheet *actionSheet = [SHKActionSheet actionSheetForItem:item];
    [actionSheet showFromToolbar:self.navigationController.toolbar];

Each sharer will get alternate text if it was set when getting "text" property of SHKItem. If no alternate text given, the property value is returned.

When it is needed. First of all - Twitter. Twitter uses markup which is useless for other places like Facebook and could look strange there. But allowing user create custom message for Twitter we simply solve the problem and bring more flexibility to awesome ShareKit.