Log Message:
Update to 0.18.  Minor changes omitted.

  - 0000473: [design] Support Preview images. (Vladimir Nadvornik)
	 - [exif] Added Nikon decryption algorithm and use it to decode
	   lens data.
  - 0000524: [design] Publish only API objects in the installed header files.
  - 0000523: [miscellaneous] C++ symbol visibility support for gcc builds.
  - 0000464: [metadata] Write support for PNG images. (Gilles Caulier)
  - 0000462: [exif] Write support for TIFF format ("New TIFF parser").