Add bower ignores

Adding back some `bower` `ignores` lost from https://github.com/components/CustomElements/blob/0.2.1/bower.json#L17-L23.

```
$ bower install CustomElements
bower CustomElements#*          cached git://github.com/components/CustomElements.git#0.2.3
bower CustomElements#*        validate 0.2.3 against git://github.com/components/CustomElements.git#*
bower classlist#*               cached git://github.com/components/classList.js.git#2013.5.14
bower classlist#*             validate 2013.5.14 against git://github.com/components/classList.js.git#*
bower MutationObserver#~0.2.0   cached git://github.com/components/MutationObserver.git#0.2.0
bower MutationObserver#~0.2.0 validate 0.2.0 against git://github.com/components/MutationObserver.git#~0.2.0
bower WeakMap#~0.2.0            cached git://github.com/components/WeakMap.git#0.2.1
bower WeakMap#~0.2.0          validate 0.2.1 against git://github.com/components/WeakMap.git#~0.2.0
bower CustomElements#~0.2.3    install CustomElements#0.2.3
bower MutationObserver#~0.2.0  install MutationObserver#0.2.0
bower classlist#*              install classlist#2013.5.14
bower WeakMap#~0.2.0           install WeakMap#0.2.1

CustomElements#0.2.3 bower_components/CustomElements
├── MutationObserver#0.2.0
└── classlist#2013.5.14

MutationObserver#0.2.0 bower_components/MutationObserver
└── WeakMap#0.2.1

classlist#2013.5.14 bower_components/classlist

WeakMap#0.2.1 bower_components/WeakMap
$ ls -al bower_components/CustomElements/
total 64
drwxr-xr-x  13 josh  staff   442B May  7 17:46 ./
drwxr-xr-x   6 josh  staff   204B May  7 17:46 ../
-rw-r--r--   1 josh  staff   685B May  7 17:46 .bower.json
-rw-rw-r--   1 josh  staff    30B May  7 13:16 .gitignore
-rw-rw-r--   1 josh  staff   105B May  7 13:16 .gitmodules
drwxrwxr-x   2 josh  staff    68B May  7 13:16 CustomElements/
-rw-rw-r--   1 josh  staff   1.3K May  7 13:16 Gruntfile.js
-rw-rw-r--   1 josh  staff   705B May  7 13:16 README.md
-rw-rw-r--   1 josh  staff   333B May  7 13:16 bower.json
drwxrwxr-x   5 josh  staff   170B May  7 17:42 demo/
-rw-rw-r--   1 josh  staff   528B May  7 13:16 package.json
drwxrwxr-x   4 josh  staff   136B May  7 17:42 src/
-rw-rw-r--   1 josh  staff   568B May  7 13:16 xtag.json
```

/cc @pennyfx