Fixes #60: Adds support for building Windows bin

[jcooklin]

Fixes #50

Summary of changes:

• Adds support for building Windows binary

How to verify it:

• run make

Testing done:

• manual

[mbbroberg]

Tested and LGTM.

during make:

2017-03-17 16:19:14 UTC [     info] building plugin: snap-plugin-collector-psutil for Windows

After:

$ tree build/
build
├── darwin
│   └── x86_64
│       └── snap-plugin-collector-psutil
├── linux
│   └── x86_64
│       └── snap-plugin-collector-psutil
└── windows
    └── x86_64
        └── snap-plugin-collector-psutil.exe

6 directories, 3 files
$ ls -lah snap*
-rwxr-xr-x  1 REDACT  REDACT   6.0M Mar 17 09:19 snap-plugin-collector-psutil.exe