Add import stats summary

This adds a `gp stats` to show the dependency import stats. This is the output:

```
calavera@dcp ~/oss/gopack (stats●●)$ ./gp2 stats
/// g o p a c k ///
updating github.com/pelletier/go-toml
pointing github.com/pelletier/go-toml at commit 23d36c08ab90f4957ae8e7d781907c368f5454dd
gopack.config missing for github.com/pelletier/go-toml
Import stats summary:

* [R] github.com/pelletier/go-toml:2
* [S] errors:1
* [S] fmt:10
* [S] go/ast:1
* [S] go/parser:1
* [S] go/token:1
* [S] io/ioutil:4
* [S] log:2
* [S] os/exec:3
* [S] os:8
* [S] path/filepath:2
* [S] path:4
* [S] regexp:1
* [S] runtime:1
* [S] sort:1
* [S] strconv:3
* [S] strings:6
* [S] testing:8
* [S] time:2
* [S] unicode/utf8:1
* [S] unicode:1

[R] Remotes, [S] Stdlib
```

It also adds tests for the current analyzer logic. Fixes #11

/cc #9