Aliased :from to :as in accessor property

When creating a document from an object the DSL language was confusing,
so this change allows you to do both:

    property :title, :from => :display_title

and:

    property :title, :as => :display_title

It would read more like: Display the property 'title' in the document as
'display_title'"
adding examples for :class and :extend lambdas, and, of course, :instance.
adding 2 test cases for xml collection, which should run
make #representer! a global test method. and use it.
XML::Collection should work now.
add Representable#read/write_fragment.
remove explicit lyrics *ahem*.
move node searching into separate method.
use XmlBinding#write in favor of #serialize_for.
using xpath for searching now in favor of search, as discussed here: http://nicksda.apotomo.de/2013/01/nokogiris-xpath-search-is-faster/comment-page-1/#comment-9403
use a simpler xpath in XML::Collection.
remove :except option.
move #write_/#read_fragment and friends to Binding.
almost finished the shiny new README.
almost finished, only some XML notes missing.
more examples.
final version of the README.
fixing README.
removing LICENSE.
remove :from occurences from README and replace with the better :as.