Don't require stdbool.h in openslide.h

fc61589 was overzealous about reintroducing stdbool.h: we can assume it
exists when compiling OpenSlide, but not when compiling programs that use
it, since those may build with MSVC.

The logic removed in that commit was incorrect, so we don't reintroduce
it here.  (It assumed HAVE_STDBOOL_H was available from someone's
config.h, and also assumed bool is 4 bytes instead of the more likely 1.)
Instead, assume for now that MSVC is the only compiler without C99
support.

Closes #115.
Update for release
Preliminary support for Ventana bif format.
Do not have official specs for file format, so this is mostly reverse engineering.

Properties:
-- Pyramidal TIFF file
-- Requires bigtiff
-- Tiles have (seemingly arbitrary) overlap

What this commit includes:
-- Identifies bif file can be opened
-- Parses metadata and populates openslide struct as appropriate.

What this commit does NOT include:
-- Accurate importing of image regions (due to non-standard overlap)
Minor bug fixes
Minor update, given up on supporting BIF format since it is proprietary.  This simply supports the Ventana TIF format now.
Merge branch 'master' of github.com:abasavan/openslide