Building base material undump script.

Based on an idea in https://github.com/peterix/dfhack/issues/119
Better way to check for the contained_items field.
Ignore Mac OS X .DS_Store files
New plugin: building-hacks. Allows workshops to block liquids and consume mechanical power, connect to machines.
Added animation and on update callback.
building-hacks: added lua convenience function, made impassible tile fix optional.
eventful: added support for custom workshops and a convenience function to interpose side panel generation
docs: updated eventful lua api and added building-hacks docs.
Minimal Readme documentation for the undump-buildings command
Merge 'ignore_DS_Store' of git://github.com/tareqak/dfhack into develop
Merge branch 'undump' of git://github.com/eswald/dfhack into develop
Updated NEWS.
Merge remote-tracking branch 'warmist/dev-buildings' into develop
Mention more clearly how the data structures are defined in the Lua API.