refactored how preference values are stored
added option to reset preferences to defaults
renamed OhmagePreferenceActivity to OhmageSettingsActivity
fixed bug preferences wouldn't show correct default value
fixed #55. added config.yaml file to define the configuration per server
changed single campaign mode to be configured in config.xml
reformatted code
fixed #56. refactored network tasks. In single campaign mode if the campaign fails to download, the user will still be logged in.
fixed crash that could happen if ResponseSyncService continues to run even if the user is logged out
fixed bug where the campaign read task would immediately start to download