Change store API to take a Reader in the Write method

The previous API took an io.Writer into the Write method, which required
closing to unlock the internal mutex. This API change is easier to lock
at high concurrency and conveniently matches the Diskv api, which is the
replacement for the FileStore internals.
Merge pull request #4 from lox/store-writefrom

Change store API to take a Reader in the Write method