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.

Finally getting my head around GitHub, This pull request is much clearer and cleaner. Sorry for the mess up with the last one.