Move CI-related files into .private directory and add unified build script

Move CI-related files into .private directory and add unified build script

The files needed for Continuous Integration (AppVeyor, Travis) are not
meant for public consumption, so clean up the root directory by moving
these files underneath .private.

Create a single build script that is leveraged by both AppVeyor and
Travis. This script replaces the previous 'travis-autogen.sh' file and
enables additional compiler warnings that should provide additional
coverage for all build environments.

Update the Travis configuration file to absorb the Brewfile and update
the Xcode images. Per warnings from Travis, Xcode6.4 is obsolete, thus
replace it with Xcode7.3. Additionally remove the "gcc" variants for the
macOS builds. The builds never used gcc to begin with and actually using
gcc causes build errors due to Clang-specific pragmas in the IOKit
header files.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>