Missing spec variable which is being created by json.load only

While running --restore-spec SOURCE spec doesn't exist so
if 'base_prefix' not in spec trow an exception
Check for tblspc_prefix path separately from path_prefix

In case of running restore on empty postgres directory we skip
creating tblspc_prefix
pep8, over indented lines
Too long line
Merge pull request #151 from bjarocki/bjarocki/restore_spec-fix

restore-spec fix