Reduce number of global variables used

This is the first step to make the exporter more testable.
Merge pull request #28 from prometheus/reduce-globals

Reduce number of global variables used
Consolidate collector selection

Remove special tags necessary for gmond and runit collectors. All
collectors get built. Selection of which collectors to use continues to
happen via parameter.
Consolidate build flags

The build of any collector can be deactivated by using "no<name>".