Correctly parse UTF-8 text in bookmark information

The modifications include a little bit more:
- The API XML is now parsed in a centralized method (APICall.parseScuttleURL), which allows to manage API-related exceptions in one place (authentication errors, etc.)
- This method uses a charset detection method (APICall.getConnectionCharset) to properly manage UTF-8 characters in XML.
- A new ScuttleAPIException is used to simplify Exception catching in calling methods
- ScuttleAddBookmark also uses the parsing method to get the result XML (it does nothing with the result code right now, but it will allow to fix https://github.com/jarofgreen/Semantic-Scuttle-Jar-For-Android/issues/1 )
- Other small clean-up, like the removal of useless imports and the removal of local username/password storing in Activity classes.
- Sorry for the big commit :(