* adding logger implementation to d2common
* Adding file loader implementation
The file loader works in terms of `Sources` and `Assets`. A `Source` is
something like a filesystem that has a cache. An `Asset` is something
that implements `io.ReadSeeker` and has a few methods of its own.
There are currently `Source` implementations for MPQ archives and for the
host filesystem, meaning that one can specify a directory on the host fs to
load files from.
`Sources` are added to a loader with `loader.AddSource(path)`, where `path`
resolves somewhere on disk. In the case that the path points to an MPQ,
then an MPQ `Source` is created and added to the loader. If `path` resolves
to a directory, then a filesystem source is added.
Files are loaded with `loader.Load("data/global/excel/monstats.txt")`, and the
sources are searched in the order that they were added.
* adding tests for d2common/logger_test.go
* adding tests and testdata for d2loader
* logger lint fixes, fixed missing test case
* minor edits, lint fixes, changes some comments, embedded Logger into Loader
* moved d2loader into d2common (I dont think it belonged in d2core)
* removed my simple cache implementation in favor of our existing cache in d2common
