Paired_with check works with multiple aliases

[KorayKirli]

for cases with 2 different aliases in filefastq aliases, create a
symlink key for the secondary aliases, and in the check, look for
symlinks and connect to the main item (first alias)

edit:

• 'files' dictionary keeps links to first alieases
• symlinks are created for rows with only paired_end information

[coveralls]

Coverage increased (+0.4%) to 86.05% when pulling 7f3f038 on bug_fixes into 2f42320 on master.