(svn r7565) -Codechange: Rework DEBUG functionality. Look for appropiate debugging levels to
use in debug.h. grfmsg() is now used as a specific debug-function for grf.

(svn r5975) -Cleanup: use ORIGINAL_BASE_YEAR & ORIGINAL_MAX_YEAR where the functions really depend on the original date format.
-Cleanup: use DAYS_TILL_ORIGINAL_BASE_YEAR where the date variables are in the old date format and conversions are needed.
-Cleanup: replace one (forgotten) instance of uint16 with Date.

(svn r5926) -Codechange: make _cur_year contain the full year, instead of the offset since 1920
-Codechange: store all year related variables that are _not_ stored in a savegame/transported over the network in the same format as _cur_year

(svn r5887) -Cleanup: move date related functions, defines and variables to date.[ch]
-Cleanup: fix whitespace related coding style issues in date.[ch]
-Cleanup: make original comments doxygen compatible and remove/change outdated comments

(svn r5346) - Feature: Add 4 new airports. 2 for aircraft, 2 for helicopters.
Commuter airport: Small. 5x4. 3 terminals, 2 helipads.
Intercontinental: massive. 9x11. 8 terminals, 2 helipads, 4 runways.
Helidepot: a small heliport with a depot for helis only.
Helistation: a large heliport with 3 helipads and a depot.

(svn r4902) Newgrf : Introduction of the newgrf_cargo files.
Clearing engine and newgrf_engine from cargo related structures.
Apart from moving stuff cargo related, there is nothing really newcargo for now.

(svn r4510) - NewGRF bounds checking:
- check Engine ID is within range
- don't try setting a vehicle name if the string id is not a valid Engine ID

(svn r4496) - NewGRF: switch custom engine names from storing a char* to using the new StringID based text system. Vehicle name
translations now work.

(svn r4486) - NewGRF: Create and use a memory pool to manage sprite groups. This
reduces the amount of house keeping we do and the chance of memory
leaks.