Add support for selecting collectors at buildtime

This works by using a global array with references to NewXCollector
functions. Each collector appends to that array in it's init() function.

Which file gets build depends on the build tags:

To build only the ganglia exporter, you can do:

    go build -tags nonative,ganglia

By default it will build only the native collector.
Add Dockerfile
Merge pull request #4 from prometheus/modularize

Modularize node_exporter and add Dockerfile