change import paths; fix `go get`

Currently this package refers to imports as a path `gokogiri/...` that doesn't exist. This makes it imposible to install as any other path (ie: "github.com/moovweb/gokogiri"). I think this can be resolved by making all local references to gokogiri components as relative imports.

ie: in `gokogiri/html/document.go` the reference to `util` should be `"../util"`

```
~$ pkg-config --cflags libxml-2.0 libxml-2.0
-I/usr/include/libxml2  
~$ go get github.com/moovweb/gokogiri
package gokogiri/html: unrecognized import path "gokogiri/html"
package gokogiri/xml: unrecognized import path "gokogiri/xml"
$ go get github.com/moovweb/gokogiri/html
package gokogiri/util: unrecognized import path "gokogiri/util"
package gokogiri/xml: unrecognized import path "gokogiri/xml"
```