feat(core): make benchmark.js a peer dependency

Closes #14

BREAKING CHANGE:
You now need to install benchmark.js yourself, it
is no longer installed by karma-benchmark.

package.json

@@ -9,13 +9,18 @@
     "Larry Davis <lazdnet@gmail.com>"
   ],
   "dependencies": {
-    "benchmark": "2.1.3",
-    "karma": "1.5.0",
     "lodash": "4.17.4",
     "platform": "1.3.3"
   },
+  "peerDependencies": {
+    "benchmark": "^2.1.0",
+    "karma": "^1.5.0"
+  },
   "devDependencies": {
+    "benchmark": "2.1.3",
     "browserify": "14.1.0",
+    "browserify-shim": "^3.8.13",
+    "karma": "^1.5.0",
     "xo": "0.17.1"
   },
   "files": [
@@ -44,6 +49,12 @@
     "pretest": "npm run build",
     "test": "xo"
   },
+  "browserify": {
+    "transform": [ "browserify-shim" ]
+  },
+  "browserify-shim": {
+    "benchmark": "global:Benchmark"
+  },
   "xo": {
     "envs": [
       "browser",