In the function "list_devices", replace grep with regex when looking for
partitions on a device.
Only the presence of /dev/sda1 has to be checked, as /dev/sda2 implies the presence of /dev/sda1.
This could be improved further by only checking if the next array element is /dev/sda1, as the array is already sorted.
Merge pull request #7 from vitamins/master

Small improvement to avoid forked grep process