Make db name-literal agnostic

Move the persistence characteristic of the store into the graph registry
and provide an API hook to get that information.

Add error return for init on a non-persistent store.

Updates #35.
Separate db.Open and db.Load

Updates #82.
Allow optional strict N-Quad parsing

This puts more of the logic in cayley, but other approaches require that
db knows about quad formats.
Merge {init,open,load}.go into db.go

The functions are too small to justify a single file each. The repl file
is left out as semantically unrelated.
Make decompressor conditional on reader interface
Add support for remote data sources

Now you can, for example:

cayley http --dbpath="https://github.com/google/cayley/blob/master/30kmoviedata.nq.gz?raw=true"
Remove now-irrelevant failing test
Merge pull request #105 from kortschak/persistence

Handle persistence and quad format in cayley