List snapshots, accept snapshot id prefix

Example:

    $ ./khepri snapshots
    ID        Date                 Source      Directory
    --------------------------------------------------------------------------------
    fa31d65b  2014-11-24 19:45:11  kasimir     /home/user/testdata
    20bdc140  2014-11-24 20:00:47  kasimir     /home/user/testdata
    326cb59d  2014-11-24 20:01:40  kasimir     /home/user/testdata
    20ff988b  2014-11-24 20:35:35  kasimir     /home/user
Add functions to compute unique prefix length
Add more checks befor decrypting data
Fix test, do not depend on snapshots command
Merge branch 'list-snapshots'
Remove race detector from tests