COMP: Add option allowing to enable internationalization support.

STATUS
 Internationalization support has been discussed during March 26th 2013
developer hangout.

 We reach consensus: Official support for i18n will be disabled. A CMake
option named Slicer_BUILD_I18N_SUPPORT will be added and the option
won't be actively supported or tested.

 Why? Supporting internationalization properly would require to test
all ITK/VTK/Teem/.. readers and we don't have the funding or resources
to support that effort. If it is something you are interested in, make
sure to drop an email on the developer list. We will be happy to provide
 more information.

DISCUSSION
 http://slicer-devel.65872.n3.nabble.com/Re-Rounding-to-integer-tt4027985.html
 http://slicer-devel.65872.n3.nabble.com/Re-slicer-users-Slicer4-can-t-really-use-it-yet-td4028040.html
 http://slicer-users.65878.n3.nabble.com/Slicer4-DICOM-many-problems-td4025919.html

ISSUES
 3029 - Image spacing not read correctly in Slicer 4.2.2