ENH: Add log file type to filepath_type

Add a filetype specific for log files. Tests have been fixed to account
for this change.
TST/ENH: Store log file generated by demuxing

When you call:

split_libraries_fastq.py ..... -o sqloutput

A file named split_library_log.txt is created, this file was previously
not being stored but it is now. The tests were modified to pass.
Merge pull request #385 from ElDeveloper/add-log-file

ENH: Save log file from split_libraries_fastq.py