Read package name from RPM header.

* Move behaviour from store to entry.
* Get rid of the Store.
* Move entry indexing responsibility into Index class.
* Move Entry and Index one level up in the namespace tree.
* Refactor parser structure, with some help from the index.
* Signature (and Header) are now internals of RPM, not dependencies anymore.
* Improve parser code a little bit. Avoid creating IO on read content by
  rewinding main IO. Duplication is still in place but I'm half a step closer
  to getting rid of it.