Added "Month to Date" Time Frame Option

Modified strings.xml to add "Month to Date" String (No translations sorry)
Modified charts_menu.xml files to Add "Month to Date" Option
Modified Prefernces.java to add "MONTH_TO_DATE" to Timeframe enum
Modified BaseChartActivity.java to include "MONTH_TO_DATE" in onCreateOptionsMenu() and onOptionsItemSelected(). Copied the functionality of the other options.
Modified ContentAdapter.java to include "MONTH_TO_DATE" to getAdmobStats() and getStatsForApp(). Using Calendar.getInstance().get(Calendar.DAY_OF_MONTH); to grab the day of the month to set again the limit.