handle directory symlinks

Our app uses directory symlinks, which anvil-cli fails to treat as symlinks (thus they are not included in the manifest).

Here's debug output from the stripped down example I was using in lieu of a spec:

```bash
$ ls -lR
total 16
-rw-r--r--  1 bob  staff   72 Nov 10 17:02 npm-shrinkwrap.json
-rw-r--r--  1 bob  staff  113 Nov 10 17:02 package.json
drwxr-xr-x  3 bob  staff  102 Nov 10 17:02 shared
drwxr-xr-x  3 bob  staff  102 Nov 10 17:02 src

./shared:
total 0
drwxr-xr-x  3 bob  staff  102 Nov 10 17:02 node_modules

./shared/node_modules:
total 8
lrwxr-xr-x  1 bob  staff  16 Nov 10 17:02 server -> ../../src/server

./src:
total 0
drwxr-xr-x  3 bob  staff  102 Nov 10 17:02 server

./src/server:
total 8
-rw-r--r--  1 bob  staff  18 Nov 10 17:02 foo.coffee
```

manifest before (missing shared/node_modules/server)
```ruby
{".gitignore-foo"=>{"mtime"=>1415667747, "mode"=>"100644", "size"=>"14", "hash"=>"bfb16fc2dc3b7f24e20981aa586e31d66293c81eb7c2e5d0a9bbb252b13c6271"}, "npm-shrinkwrap.json"=>{"mtime"=>1415667747, "mode"=>"100644", "size"=>"72", "hash"=>"796f13c4702a469b7799e962aed4952f30570ac51c33cb33fab33f4fca4e4c11"}, "package.json"=>{"mtime"=>1415667747, "mode"=>"100644", "size"=>"113", "hash"=>"ec8efb25ada9845df2ea0834ba504e63aa88b50273f404dbf2d966edf0103991"}, "src/server/foo.coffee"=>{"mtime"=>1415667747, "mode"=>"100644", "size"=>"18", "hash"=>"850d92c5b96cad1033c33f413e9795f1af842abc000d52de6319426619d14da3"}}
```

manifest after
```ruby
{".gitignore-foo"=>{"mtime"=>1415667747, "mode"=>"100644", "size"=>"14", "hash"=>"bfb16fc2dc3b7f24e20981aa586e31d66293c81eb7c2e5d0a9bbb252b13c6271"}, "npm-shrinkwrap.json"=>{"mtime"=>1415667747, "mode"=>"100644", "size"=>"72", "hash"=>"796f13c4702a469b7799e962aed4952f30570ac51c33cb33fab33f4fca4e4c11"}, "package.json"=>{"mtime"=>1415667747, "mode"=>"100644", "size"=>"113", "hash"=>"ec8efb25ada9845df2ea0834ba504e63aa88b50273f404dbf2d966edf0103991"}, "shared/node_modules/server"=>{"mtime"=>1415667747, "mode"=>"40755", "size"=>"102", "link"=>"../../src/server"}, "src/server/foo.coffee"=>{"mtime"=>1415667747, "mode"=>"100644", "size"=>"18", "hash"=>"850d92c5b96cad1033c33f413e9795f1af842abc000d52de6319426619d14da3"}}
```