Add `ledger-read-date' - Read date & default to today.
`ledger-insert-effective-date': use `ledger-read-date'.

Running `ledger-insert-effective-date' will now prompt for a date with
a pre-filled value of the `(ledger-year-and-month)' ("2014/01/"). Just
pressing RET without inserting the 'day'-portion will insert the date
of today.
ledger-reconcile: Add option to insert an effective date when clearing transactions.
`ledger-read-date': Use `ledger-minibuffer-history'.
Use `ledger-read-date' where applicable.
Merge pull request #228 from the-kenny/emacs-prompt-effective-date

Emacs: Improve effictive-date-handling