Add to the Windows build script a clause to create and use a virtual
environment and install Sphinx there, comparable with the Unix Makefile.
The user may suppress this behaviour by defining a her preferred command
in the environment variable SPHINXBUILD, as before.

* Build in a virtual environment on Windows (GH-267)

Add to the Windows build script a clause to create and use a virtual
environment and install Sphinx there, comparable with the Unix Makefile.
The user may suppress this behaviour by defining a her preferred command
in the environment variable SPHINXBUILD, as before.

* Simplify wording and add comment following review.

* Always build relative to the directory containing make.bat

Whatever the current directory when invoked, the build takes place into
_build/... in the directory that contains make.bat, and returns to the
invocation directory.

* Properly escape parentheses in the help text.

Previously this disabled subsequent build targets by making "goto end"
accidentally unconditional. We also describe the "check" target.