Issue #100: Made a few changes that were found during review of PR #103.

The following items were changed:
  * Removed commented options to sync on a specific day of the week and sync on
    a specific day of the month.
  * Added additional possibilities to sync every: 5 minutes, 4 hours, daily (not
    at a specified time), and weekly.
  * Added some summary text to the list preference in the SettingsActivity that
    displays the current sync setting, so the user doesn't have to click into
    the sub-screen to see which sync setting is selected.