Handle 'cwd' option in .bowerrc

I have following project structure
```
project
  server
    ...
  client
    bower_components
    ...
    bower.json
  .bowerrc
  gulpfile.js
```
I've set 'cwd':'client' in .bowerrc and bower is working fine from project directory

Now i need that gulp tasks also have access to installed components
You should change the cwd if at least one the both options is set:

```js
if (config.cwd || config.directory) {
    cwd = path.dirname(bowerrc);
}
```