Changelog for LedgerSMB v 2.6.17

(Changes relative to the pre-fork SQL-Ledger 2.6.17)

* Corrected sessionid security hole allowing bypass of login to main application
* Corrected sessionid security hole allowing one to list logins and more.
* Changed acc_trans.amount to NUMERIC
* Tightened browser caching rules to prevent problems with back button.
* Added an open content manual to the main distribution.
* New logo. 
* Began whitespace reformatting of main application.