Fix for PEP8's E265.

pep8 1.5 introduced a new test (E265) to ensure there's at least one
space before block comment.
Use @property to create the os attribute in Image

Use the @property decorator instead of the property() method to create a
read-only os attribute in Image class.
Add device attribute in Disk class

This attribute hosts the block device associated with this Disk
instance. This can be the original media if the source media is a block
device, a loop device of the original media if the original media is a
regular file or a loop device of a temporary object if the original
media is a directory.
Add REG_{SZ,DWORD,BINARY} helper funcs in Registry

Those lambda functions are used to set a value of type REG_SZ,
REG_DWORD and REG_BINARY in a registry node
Extend SysprepParam class to include a value check

Extend the SysprepParam class to support defining a custom check
function which can be used to check if the value given by a user when
updating a sysprep parameter is valid.
Merge PR #16 (feature-code-cleanup) from skalkoto/snf-image-creator

Small fixes and code beautification.