Fix period expression parsing for month to month without year

Bug fix for #1074, a regression introduced by the fix for bug #375
Add ISO 8601 date as recognized date string

Dates specified via --begin and --end are converted to a value expression
using an ISO 8601 (yyyy-mm-dd) date, but this date was not recognized by
ledger.

Bug fix for #1072
Update doc/NEWS for ledger 3.1
Bump version number to 3.1