Added an option (on Windows) to use prebuilt binaries.

This defaults to ON in order to preserve existing behavior.
Only force Inchi off if using prebuilt binaries.

The default behavior is still preserved, and the logic simplified. Inchi
can now be build for MSVC too, if the user chooses to.
openbabel-csharp.i: Fix Downcast method (was already fixed in 2.2.x but was not merged to trunk)
pybel.py: Updated to synch with Cinfony.
scripts/openbabel-perl.i: Fix warning
src/ops/opisomorph.cpp: Fix warning
a_obpython.bat: Minor correction to setup for generating of Python bindings on Windows.
OBGroupContrib: Ensure all exported API cpp code is inside the openbabel
library.
Add unit test for writing stereo bond flags.
Ignore compiled subdirectories.
SVG format: Use thicker lines
SVGFormat: Add -xs option to enable asymmetric double bonds. This option
is not enabled by default.
SVGFormat: Make thicker lines optional with -xt option. This options is
not enabled by default.
Remove invalid stereo bond unit test
Revert "OBGroupContrib: Ensure all exported API cpp code is inside the openbabel"

This reverts commit affc33b86be0a50a0d1c8b80a1b1c3f2b7000108.
Fix bug with sp3 atoms -- now properly defines 109.47 degrees for NH3, CH3, etc.
Add improved symmetry detection and Symmetrize() function.
Add AliasData cast for Igor.
Fix warning from clang-analyzer (i.e., use "count" instead of discarding.)
SVGPainter & SVGFormat: Enable black background again.
Read and write valence field in MDL atom block when it is 15 (no attachments)
Do not process the output mol vector in deferredformat if it is empty