Fixing sychronization and concurrency issues (#123)

* Improved presentation of errors if login (SettingsActivity) or synchronization fails.

* Show icon if a note is not synchronized (hint to a possible error)

* Refactoring: move common code to new private method getNotesRawQuery(String sql, String[] selectionArgs)
New method getLocalModifiedNotes() is a preparation for bugfixing #117

* last part from the previous refactoring

* use material design icon and remove old holo icon

* Refactoring NoteSQLiteOpenHelper and NoteServerSyncHelper in order to fix several bugs belonging concurrency and synchronization. Outstanding: documentation, testing, cosmetic changes

* Show icon if a note is not synchronized (hint to a possible error)

* Refactoring: move common code to new private method getNotesRawQuery(String sql, String[] selectionArgs)
New method getLocalModifiedNotes() is a preparation for bugfixing #117

* last part from the previous refactoring

* use material design icon and remove old holo icon

* Refactoring NoteSQLiteOpenHelper and NoteServerSyncHelper in order to fix several bugs belonging concurrency and synchronization. Outstanding: documentation, testing, cosmetic changes

* only pull remote changes if this was demanded by the caller

* restructure the communication between synchronization task and user interface using callbacks, only try to sync if not offline, otherwise show an error message.

* updateNoteAndSync(): only make database changes, if the content really changed (see #104)

* javadoc and cleanup

* Bugfix for EditNoteActivity: invoke callbacks directly if note wasn't changed in edit (before, callback was never invoked in this case)

* Make sure, that saveDataWithUI is not called, when the previous saveAndSync is still running. In addition, start next saveAndSync not before a small delay has passed.

* rename auto-sync method; adjust DELAYs

* Quick Bugfix: Toast in AsyncTask have to be in onPostExecute

* Bugfix: save edited note in offline mode, too

* Bugfix: Show error message in UI thread

* reduce writes to local storage

* reduce logging

* Refactor: remove recurrent code

* UI enhancement when editing notes (with no changes)