Add text(), find(), and allow DOMNode insertion.

- You can now use Nekomata->text() to insert any text without having to
  create an element first.

- Nekomata->find() is added mainly for solving the issue of
  SimpleXMLElement unable to locate a namespaced node and SXE's xpath
  always returns an array even if there's only one node found.
  Chaining xpath call is impossible unless you're using php 5.4.0+
  (with array dereferencing).

- Replaced Exception with DOMException

- It is now possible to pass an instance of DOMNode into Nekomata->_().
  You can add node types that aren't supported by Nekomata with this.
  For example:
      $doc->_(new DOMCDataSection('console.log("foo")));