System default emojicon

1. Allow for using system default emojicon by returning immediately in addEmojis() if useSystemDefault
 * useSystemDefault parameter is passed all the way from UI components to EmojiHandler
 * Old API's are all retained via method overloading
 * Re-sizing and Softbank emojis are not handled if useSystemDefault, otherwise it wouldn't be system default at all
2. Various changes to example app adopting to useSystemDefault parameter
 * However calling useSystemDefault setters in EmojiconTextView/EditText does not trigger the change in text appearance, XML attributes do work though 
3. Lower min sdk version requirement to 7 (because I need 7 in my own project :-)
Is this change necessary?