Add Variable env attribute

The env attribute is used to mark variables that should be exported to shell
environment.

Example (recipe syntax):

    FOO = 'bar'
    FOO.export = True

Will cause FOO variable to be set as shell environment (and export'ed)
variable in task Docker containers.

Signed-off-by: Esben Haabendal <esben@haabendal.dk>
Add support for raw/non-wrapped Namespace.eval()

Signed-off-by: Esben Haabendal <esben@haabendal.dk>
Add basic Task variable support

Signed-off-by: Esben Haabendal <esben@haabendal.dk>
Add extended Task variable attributes

Proper testing of them in the unittest is not implemented yet.
Might be better to do in combination with the Task class.

Signed-off-by: Esben Haabendal <esben@haabendal.dk>