create PWD test cases

test the initial CWD

fix asylumfunk#15
reject parameters to PWD command

add corresponding tests

fix asylumfunk#16
reply 502 to unsupported commands

add tests for all unsupported RFC959 commands

fix asylumfunk#17
Merge pull request #15 from asylumfunk/issue/15-16

add PWD testing and strict RFC interpretation
Merge pull request #16 from asylumfunk/issue/17

reply 502 to unsupported commands