Simple LevelDB sample dumper tool.

This very simple tool dumps all samples in a LevelDB metrics storage
together with their chunk information. This is very crude for now - I
just whipped it up because I needed to debug something. We should add
printf-style formatting support for it in the future to indicate what
information to output for each sample in the database. We could also
support inspecting the index databases.
Merge pull request #266 from prometheus/tool/dumper

Simple LevelDB sample dumper tool.