Add support to select gzip or bzip2 (default) as compression method

This pull-request adds an argument -f which can be set to "gzip" or "bzip2" while creating an image.

for example:
`tpl -f gzip -b 9eac5c0c-a941-11e2-a7dc-57a6b041988f -z 991e1640-d8f3-11e2-b7ca-9fde7e2f67c6 mi-example`