Add Read instance

* Match `Show` instance to `Data.Sequence`
* Use "fromList" instead of "Data.DList.fromList" to match style of other
  modules (`Data.Set`, `Data.Sequence`, etc.)
* Add properties for `read . show` and `show . read`
Add Foldable and Traversable instances
DEPRECATED: concat, foldr, map
Change to Markdown and simplify
Scripts to run HPC (program coverage) and clean up

Something is apparently broken with hpc as used by cabal-install:

```
$ cabal configure --enable-library-coverage --enable-tests
$ cabal build
$ cabal test
[...]
Warning: Your version of HPC (0.6) does not properly handle multiple search
paths. Coverage report generation may fail unexpectedly. These issues are
addressed in version 0.7 or later (GHC 7.8 or later). The following search
paths have been abandoned: ["dist/hpc/mix/dlist-0.5"]
hpc: can not find dlist-0.5/Data.DList in ["./dist/hpc/mix/test"]
```