#1 Extract API and Doc Improvements

[Daniel Doherty]

I posted these on comp.lang.ruby, but realized that this is
probably the better place for these.

1. It would be really helpful to see an example of
   iterating through a zip archive and extracting all the
   entries. The documentation appears to lack one for this
   very common unzip operation.

2. A design issue. I would have thought it natural for
   "extract" to default the second argument to "./entry.name"
   so that in the most common case, it would not need to be
   supplied.

3. Another design issue. It would seem more natural to
   me to make "extract" a method of the Zip::ZipEntry class,
   which would defult to extracting itself into the current
   directory with the entry name. That way, an unzip
   operation would become:

Zip::ZipFile.open("archive.zip").each { |entry|
entry.extract

}

Extraction now requires an awkward reference to the
Zip::ZipFile object
inside the block.

1. The extract method should have a parameter that
   specifies whether over-writing an existing file should
   occur. In my opinion, it should default to overwriting, but
   I'll concede that that may be the wrong choice.

My two cents. Thanks for the great library, and keep up
the good work.