rewrite how we handle document
DocumentFile: constructor throws FileNotFoundException
use exception to check file format validity
no need for this
fix error after rebasing
move PostList and PostListEntry to index package
Merge branch 'doc'

Conflicts:
	src/main/java/com/mee/ir/index/IndexWriter.java
	src/main/java/com/mee/model/Document.java
fix compilation error after merge