Create core directories if they do not exist.

[mohitranka]

Currently, if the directory does not exist for mutation it throws an error, and quits.

./dgraph --mutations /dgraph/m --postings /dgraph/p --uids /dgraph/u  
2016/07/12 15:55:34 num_cpu: 4. prev_maxprocs: 4. Set max procs to num cpus  
2016/07/12 15:55:34 Unable to find dir: stat /dgraph/m: no such file or directory  

Dgraph can instead create the directories and give a slightly better experience.

Thoughts?

[mohitranka]

Case in point: #136

[manishrjain]

Makes sense, should be relatively trivial to add. Do you want to send us a PR, @mohitranka?

[mohitranka]

Merged to develop with commit 9d91939