Documentation

I HAVE PUSHED TO "MASTER" (I AM NOT SURE IF I HAVE TO PUSH TO NEXT OR MASTER, MASTER IS DEFAULT PROPOSED BY GITHUB)

The main changes are:

ledger3.texi
- Add description for --time-colon
- Add description for --unrealized-gains and --unrealized-losses 

ledger-mode.texi
- Add indexes (Concept, Command, Variable, Keystrokes)
- Added Copying Transaction section

ledger3.texi & ledger-mode.texi
- Fix "copying" section