Move code out to exometer_core

This is a fairly major change - feedback wanted.

The exometer repository has grown quite complex, as demonstrated by the need for custom package management to limit dependencies. In order to make it easier to use the core services of exometer, I created a new repository, [exometer_core](https://github.com/Feuerlabs/exometer_core], which exometer uses as a dependency. The `exometer_core' repos mainly contains everything except custom reporters.

Some details still need to be fixed:
* The doc overview of exometer needs to be updated to describe the new structure
* When checking env variables, exometer_core should allow `exometer' env vars to shadow `exometer_core' env vars - the major reason being that legacy code setting exometer env vars should get the same behavior as before.

Please report any compatibility issues and/or good reasons for not making this change (+1 votes are also welcome).