Added -fvisibility=hidden parameter for gcc.

Should help a bit with endless missing DLL_LINKAGE attributes and
(according to gcc docs) help a bit with library loading times.
fix mingw build
(?) does this brake *nix build?
First part of object instance API cleanup

- removed passability() method in favour of passableFor(PlayerColor)
- moved operator < code to map handler
- updated class documentation
Moved FoW management from CGObjectInstance to callback
Refactoring of hoverText from objects:

- removed hoverText field
- split getHoverText() method into 3:
- - getObjectName() for generic name
- - getHoverText(Player) for player-specific text
- - getHoverText(Hero) for hero-specific strings
Minor fix to schema, server will print visited objects in debug log
Ugly workaround for DLL_LINKAGE issue. Should be removed once I figure
out what's triggering it.
Reduced number of #include's in headers. May break compilation on Win
since some net packs now need DLL_LINKAGE
- H3M parser will create objects that don't have data in map file via
object handler.
- Added large number of missing objects to config
Merge pull request #1 from vcmi/develop

Merge upstream changes