added support for gzipped image devices

You can now supply *.tar.gz files as image device
The server will inflate the file on the final pipeline stage, before passing the image.tar to zerovm session

To signal that the gzipped file is used, you must set content type to "application/x-gzip" when uploading such files to Swift

Please note that setting "content_type" property in the json servlet file will have _no effect_ , and will lead to error if the actual content type on Swift object is not "application/x-gzip"
added error handler for bad gzipped data
added ability to send tar.gz as POST payload

Now users can send gzipped tars as a payload to executable POST request
tar stream will be inflated on-the-fly at the proxy entry point

Some effort was made to never use more than two buffers of network_chunk_size for inflation process.
See gunzip_iter() implementation.
That will (hopefully) avoid zip-bombs.
Merge pull request #18 from pkit/gzip_support

added support for gzipped image devices