Added Python2 and Python3 Scripts dir to PATH using installer option (#111)

[obilodeau]

This allows pip installed programs that provide scripts to be accessible from the cmd.exe. This is how this works on Linux.

Example without this option:

pip3 install git+https://github.com/GoSecure/malboxes.git@pip-packaging#egg=malboxes
C:\...> malboxes -V
'malboxes' is not recognized as an internal or external command,
operable program or batch file.

Example with this option:

pip3 install git+https://github.com/GoSecure/malboxes.git@pip-packaging#egg=malboxes
C:\...> malboxes -V
malboxes-script.py 0.2.0dev

[obilodeau]

Added Python2 after discussion in #111.

I tested the Python2 solution and it worked for me on Windows 10 32 bit using python2-x86_32.

[gep13]

@obilodeau looks good to me. Thanks for getting this done, and apologies for taking so long to get back to you about this.