Allow storage of persistant per-process lager metadata

For persistant processes with some immutable metadata (riak vnode and
the vnode ID, for example), implement lager:md/0 and lager:md/1 for
getting/setting such metadata into the process dictionary.

Such metadata is automatically included in any lager message metadata,
so you can just set it in your init() function or whatever and not have
to worry about passing the data around and using it in every lager call.
Merge pull request #121 from basho/adt-pdict-metadata

Allow storage of persistant per-process lager metadata
Add colors to lager, not enabled by default
Merge pull request #124 from DeadZen/dz-lager-colors

This change implements a color spectrum for severity levels, disabled by default.
Tell travis to test R15B03 and R16B as well