First cut package for nextflow, has issues

This is my first cut at a package to support nextflow.  It's also my
first package.  It works, but has issues.  I'm going to submit a pull
request and get some coaching on how to deal with it.

One issue particular: if I install, then uninstall, then try to install
again (which uses the cached copy of the "distribution file"), it
explodes.

var/spack/repos/builtin/packages/nextflow/package.py

@@ -0,0 +1,24 @@
+from spack import *
+from glob import glob
+import os
+
+class Nextflow(Package):
+    """Data-driven computational pipelines"""
+
+    homepage = "http://www.nextflow.io"
+
+    version('0.20.1', '0e4e0e3eca1c2c97f9b4bffd944b923a',
+            url='https://github.com/nextflow-io/nextflow/releases/download/v0.20.1/nextflow',
+            expand=False)
+
+    depends_on('jdk')
+
+    def unpack(self):
+        pass
+
+    def install(self, spec, prefix):
+        chmod = which('chmod')
+
+        mkdirp(prefix.bin)
+        install("nextflow", join_path(prefix.bin, "nextflow"))
+        chmod('+x', join_path(prefix.bin, "nextflow"))