Re #8882. Enable script to be run if called from another directory.
Re #8882. Use the NSIS installer by default and always.

The MSI installer is long-dead so we don't need to leave the option
hanging around (especially as the default). The -v option is still left
in the list of possible flags for backwards compatibility, but it does
nothing.
Re #8882. Generalise the path to the stresstesting module.

So that this script can be run from a different working directory.
Re #8882. Forgot the import.
Re #8882. Also forgot a directory separator.
Re #8882. Set DYLD_LIBRARY_PATH on the Mac.

This enables mantid to be imported into python.
Merge remote-tracking branch 'origin/feature/8882_jenkins_build_scripts'