NightlyBase doesn't convert string in dayOfWeek param

No conversion is made of the value in dayOfWeek if it's a string. The
value is directly passed to croniter without proper conversion.
This fix handles both single string values and multiple comma separated
values as well as abbreviation in the string. ("0" "5,6" "0,fri" et.c.)
Updated variable name and comments
Merge pull request #1424 from gangefors/fix/nightly_doesnt_convert_string_in_dayofweek

NightlyBase doesn't convert string in dayOfWeek param
Increase unittest coverage for getChangesForBuild
Fix bugs raise by the unittests when several codebases are used