emacs-mode: Prefix every function/variable/macro with ledger-.

This is the common convention in Emacs Lisp. Not following it might
cause problems with other packages as functions/variables/macros are
always visible to other modules.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>
ledger-mode: Add `ledger-remove-effective-date'.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>
Improve `ledger-insert-effective-date'.

The function now replaces already existing effective dates in the same
line.

With a prefix (C-u C-c C-t), remove the current effective date.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>
ledger-mode: Introduce `ledger-default-date-format'.

Some elisp functions depend on a date-format supplied in ~/.ledgerrc. Change these by introducing a default ("%Y/%m/%d").

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>
Merge pull request #226 from the-kenny/emacs-bugfixes

Bugfixes & Improvements for Emacs Support

Thanks!