hook new Image endpoint into main Linode object

The `linode.image` namespace was missed in https://github.com/rick/linode/pull/23

```
displague@i:~/src/linode$ bundle exec irb -I lib
irb(main):001:0> require 'rubygems' ; require 'linode'; 
l = ::Linode.new :api_key=> 'abc123'
=> #<Linode:0x00000001e5ef60 @logger=nil, @api_key="abc123">
irb(main):002:0> l.image.list
NoMethodError: undefined method `image' for #<Linode:0x00000001e5ef60>
	from (irb):2
	from /usr/bin/irb:11:in `<main>'
```

The tests don't catch this because they jump right into the Linode::Image class, without using the Linode namespaces.

With this PR:
```
irb(main):005:0* l.image.list
=> [#<Linode::OpenStruct last_used_dt="", minsize=1, description="test description imageid1", label="test label imageid1", creator="displague", status="deleted", ispublic=0, create_dt="2014-09-03 00:53:21.0", type="manual", fs_type="ext3", imageid=9610>, ...]
irb(main):006:0> l.image.delete :imageid => 9610
=> [#<Linode::OpenStruct last_used_dt="", minsize=1, description="test description imageid1", label="test label imageid1", creator="displague", status="deleted", ispublic=0, create_dt="2014-09-03 01:17:27.0", type="manual", fs_type="ext3", imageid=9610>]
```

I see the need for testing that namespaces exist within Linode, but I'm not sure how you would approach this.  
Check for the existence of the tested item within `Linode` from each `spec/linode/*_spec` ?  Or would you have `linode_spec` check all known names (seems pointless) or derive the names from a directory listing?