Attributes are now vectors, not maps.

While attributes in most cases may occur only once, there are special
cases like #+PROPERTY: where the same attribute may be used repeatedly.
Move attribute handling into its own class.
Implement Attributes::drawerNames.

The results of the first parser pass will be stored in a temporary
OrgFile in the parser. Then the regular methods to extract data can be
used, instead of embedding knowledge about it in the parser code.