Read list of installed files.

Also, change assertions to expectations in previous tests.
Hide some tiny bit of information in Entry.

It turns out that not all entry fields are needed to be exposed to the world.
Refactor relationship between RPM and Header.

There's still something that boggles me a bit, but it's not clear what that is
yet (though, it feels much better now than before). For the moment,
responsibilities are broken out as follows:
* Header: maps access to the index by tag to proper methods
* RPM: transform header data into useful, more friendly data/abstractions
Simmetrically refactor relationship between RPM and Signature.

As the previous commit, responsibilities are now partitioned as follows:
* Signature: map access to index by tag to meaningful methods
* RPM: provide useful abstractions around RPM signature data
Improve documentation.