small changes that got me started

1. Wouldn't compile on my Mac until I made it compatible with `go` 1.3. I guess Macports is behind the times.

2. I spent ten minutes figuring out why `"p1m"` was getting lost when the variable `piFile` was passed from `pisearch_test.go` to `pisearch.go:Open`.. then I realized that only the suffix was hardcoded in the message. Enhance the error message.

3. Clarify where the `pi1m.4.{bin,idx}` files go in `README.md`. (Though the answer may be obvious, I did it wrong the first time. :)

Criticize wildly; I've never used go before.