Change API: Container => File.

Rename ZipContainer::Container to ZipContainer::File. This makes more sense as
we are dealing with actual file serializations of containers. It also matches
the rubyzip API more closely which will become even more obvious when other
rubyzip API entry points (OutputStream, InputStream) are implemented.

(cherry picked from commit f0ad8d73d6a0cc6e190841ba91a4663bc3c1956a)

Conflicts:
	ReadMe.rdoc
	lib/zip-container.rb
	lib/zip-container/file.rb
	test/tc_create.rb
	test/tc_read.rb