[gen] add script to generate directory indexes
[gen] add option to '--ignore' directories
[gen] use git log to retrieve the last modification date

Using 'stat' to retrieve this information cause generated index.html
files to be updated each time the build is triggered.

Files generated by command
    git clone git@github.com:dudie/maven-repository
have a modification time of 'now', different each time the index.html
files are generated.
[travis] enable travis-ci