add artisanal handcrafted SVG Spack logo. (#6165)

- This isn't one of those autogenerated SVGs from a drawing program!

- This is a completely re-traced, minimalist SVG file with clearly
  delineated pieces so that your favorite renderer can draw a Spack logo
  at whatever resolution you want.

- Included versions with text, as well.

README.md

@@ -1,5 +1,4 @@
-![image](share/spack/logo/spack-logo-text-64.png "Spack")
-============
+# <img src="https://cdn.rawgit.com/spack/spack/features/svg-logo/share/spack/logo/spack-logo.svg" width="64" valign="middle" alt="Spack"/> Spack
 
 [![Build Status](https://travis-ci.org/spack/spack.svg?branch=develop)](https://travis-ci.org/spack/spack)
 [![codecov](https://codecov.io/gh/spack/spack/branch/develop/graph/badge.svg)](https://codecov.io/gh/spack/spack)