Add Two new methods for metadata retrieval

[thomas694]

Metadata can be retrieved with ReadCentralDir.
The data for newly added files is appended to the stream, but the central directory information is not updated until the archive is closed. Until then, the ZipFileEntry for a newly added file is just added to an internal list.
As ReadCentralDir should remain untouched, it would be nice to have two methods GetEntries and GetEntry to retrieve the entire list or search for a specific entry.

The new version has two methods that evaluate both metadata sources to prepare the result.

Fixes #58

[thomas694]

After merging PR #61:
zip.ReadCentralDir();
can be changed to:
zip.ReadCentralDir(true);