BUG: Add support for signed char output

This patch addresses the following JIRA ticket:
https://issues.itk.org/jira/browse/SIMPLEITK-393

The types "char", "signed char" and "unsigned char" are distinct
intrinsic types. And char may be signed or unsigned. It has been
generally assumed that ImageIOBase::CHAR refers to a signed
type. To support writing of "int8_t" types, the correct typemap has
been added.

Change-Id: If4afaf277cf98d83d9a007b122631ab2aea4a647
Merge topic 'OptionalSupportSignedCharImageIO'

70c8a7da BUG: Add support for signed char output