vmnetfs: Permit images to start at offset > 0 on the server
Add initial remote zipfile code
Store all components of a VMNetX image in a single package file
Convert package paths to file URLs in MachineMetadata
Perform HEAD request in _HttpFile constructor

ZipFile's first I/O is a seek relative to EOF, so in practice we will
start with a HEAD anyway.  This simplifies _HttpFile and allows Package
to detect HTTP errors during the _HttpFile constructor rather than
receiving a mysterious BadZipfile exception later.
Add XML reference documents that point to a remote VM package

To avoid redirect loops and malicious filesystem access, reference
documents are only respected if they come from the local filesystem,
and can only point to a package, not to another reference.
Update MIME type registrations

Add application/x-vmnetx-package for VMNetX packages.

Map .netx files to application/x-vmnetx-reference+xml.
Update README for single-file packages
Allow "fh" as an argument name
Package doesn't need to store the ZipFile as an attribute
vmnetx.generate: Add compress_machine() utility function

Closes #19.
Raise custom exception from _HttpFile

The contract of a file-like object requires it to raise IOError.
However, ZipFile converts this to a BadZipfile and swallows the error
message, making debugging impossible.  Raise a custom exception instead.