Add "conditions" optional Array option, for predefined custom S3 Policy "conditions"

Can be used to add _other_ "conditions". For example, to allow the `Cache-Control` header to be passed in, then you would do something like:

``` js
var policy = require('s3-policy');

var p = policy({
  secret: 'something',
  bucket: 'i.cloudup.com',
  key: 'asdfasdfaewrw',
  acl: 'public-read',
  conditions: [
    ['starts-with', '$Cache-Control', '']
  ]
});
```

Future thoughts: we could potentially add a `headers` Object option, which would streamline the above logic, however the list of allowed headers is very small, so it might not be worth it.