bugfix,init page object's lexed attribute.

the page object's attribute 'lexed' is not correctly inited.so when
use page.lexed to generate the search index file,nothing will be
indexed.
Merge branch 'master' of github.com:GitbookIO/gitbook into fork

Conflicts:
	lib/parse/page.js
Merge pull request #489 from codepiano/fork

bugfix,init page object's lexed attribute.