Add support for other paths for ipfs binary

Use `which` to locate `ipfs` rather than guessing.

lib/ipfs.js

@@ -1,9 +1,14 @@
 require('shelljs/global');
 
 ipfs = function(build_dir) {
-  ipfs_path = "~/go/bin";
+  ipfs_bin = exec('which ipfs').output.split("\n")[0]
 
-  cmd = ipfs_path + "/ipfs add -r " + build_dir;
+  if (ipfs_bin==='ipfs not found'){
+    console.log('=== WARNING: IPFS not in an executable path. Guessing ~/go/bin/ipfs for path')
+    ipfs_bin = "~/go/bin/ipfs";
+  }
+
+  cmd = ipfs_bin + " add -r " + build_dir;
 
   console.log("=== adding " + cmd + " to ipfs");