smpp_time.py: rewrote parse_relative_time() and unparse_relative_time() as they would not parse relative time when months==00 or days == 00

smpp_time_test.py: created a new test case for relative time with months=0 and days=0
Merge pull request #15 from dliappis/master

smpp relative time parsing improvements
Removed tests from gitignore
Added credits
Update README.markdown
Merge from mozes/smpp.pdu
Merge branch 'master' of github.com:rtrdev/smpp.pdu into rtrdev-master
Added thx to rtrdev
Travis CI setup
Merge with incoming changes