force cwd to be an absolute path

fix asylumfunk#19
refactor USER and PASS

declare vars immediately with narrowest scope
use reply messages from RFC959
remove superfluous comments
remove duplicated logic/tasks

fix asylumfunk#20
accept PASS only immediately after USER

fix asylumfunk#21
branch explicitly

whitespace changes due to indentation
update PWD tests

test for empty initial CWD values
remove trailing slash from test cases
Merge pull request #17 from asylumfunk/issue/19

ensure intiial CWD is always an absolute path