ENH: Add option ENABLED_SHARED to itk_module for shared libraries

Bump required cmake version to 2.8.6 for GenerateExportHeader.

This option enables shared libraries on windows and adds export
specification with gcc shared libraries. This option respects the
CMake variable BUILD_SHARED_LIBS.  Modules without this
specification can still be shared with gcc, but will be static with
msvc.

Change-Id: Ic1552ca64bb7e614e254cefe285802c96359ed3f
ENH: Enabling shared IO modules on windows

Adding module specific export specifier for ImageIO modules which do
not depend on third party libraries. These io modules are enabled to
follow the BUILD_SHARED_LIBS cmake variable. This enables these
libraries to be shared on windows, and have specified export
visibility with GCC.

Change-Id: I804a2a8fc7fd6afe3f12fa05b9c8858bbddae0a4
Merge topic 'ImageIOSharedLibraries'

e55e67b ENH: Enabling shared IO modules on windows
6d369e1 ENH: Add option ENABLED_SHARED to itk_module for shared libraries