Added teacher mode

Teachers can now see their own table, which has a different data structure. Therefore a separate SubstitutionTable class is needed. Teacher mode is detected by comparing the username string with "Lehrer".
Fixed display of HTML line breaks in daily information

HTML line breaks (<br>) are stripped out by node.text(), therefore texts of all TextNodes are joined with a '\n' as separator (no '\n' at the end).
Added hint about account information in AuthenticatorActivity
Fixed broken width determination on orientation change

Width determination is used for displaying the substitution information only when there is enough space (avoids horizontal scrolling). The determination will fail if the device is rotated because the activity's layout has not been fully created and therefore it will return wrong values.
The fix determines the device's screen width on app startup by using a dummy activity (MeasureActivity) with a custom view. This dummy activity will start the MainActivity immediately.

Furthermore, bump version.