Added shareToMail and shareToTwitter, also changed shareToFacebook

[mattlanham]

• Now uses documented way to target specific services
• Added shareToMail and shareToTwitter

[max-mapper]

@mattianham trying to update this for phonegap 1.2.0. SBJSON is now gone (replaced by JSONKit), but I dont see where it was used in this plugin before anyway. I deleted the header include lines for SBJSON and the plugin builds and runs okay, but I havent tested it with all of the different sharing options yet.

Do you think its safe to remove the SBJSON include? https://github.com/phonegap/phonegap-plugins/blob/master/iPhone/ShareKitPlugin/ShareKitPlugin.h#L16

[max-mapper]

@mattianham also it looks like ShareKit hasnt integrated the iOS5 Twitter SDK yet, but you can use it via the Twitter plugin in this repo FYI