Make the Watcher use util.readFileP for reading files.

Now there are no direct uses of fs.readFileP other than in util.readFileP.
Limit the number of simultaneous active fs.readFile calls.

This goes a long way towards avoiding EMFILE errors (too many open files), though fs.watch still contributes to the problem.

Closes #22.
Bump NPM version to 0.6.7 in preparation for publishing.
Merge pull request #23 from benjamn/readFileP-batching

Limit the number of simultaneous active fs.readFile calls