Use a better runtime dir

We were using ./data, which doesn't make sense as we don't know the
current directory.

Instead, we now use the XDG_RUNTIME_DIR as defined by Freedesktop.org.

This was worked on with Tommy Cheng, Koala Yeung and Mathieu Bridon.
Fallback on /tmp for the runtime dir

Modern Linux systems should have the XDG_RUNTIME_DIR environment
variable set.

This lets us fallback on /tmp for all other systems.
Merge pull request #8 from bochecha/master

TEMP FIX. I will change the path to sth like mysql and pgsql using.