This commit refactors the config into a `main.Config` object instead
of a global singleton. The `core.Service` is also refactored into
the `main` package and individual pieces of the service are wired
together by the `pilosa` binary.
These two changes are required to begin to decouple packages from
one another and allow them to be individually unit tested. Previously
most top-level objects in the system could access any other top-level
object through the `core.Service` which effectively made `Service` a
singleton in the system. Each top-level object now has inline interfaces
for their dependencies so that can be set at runtime by the `main`
package or can be mocked by a test package.
This commit updates the import path from:
```
pilosa/*
```
to:
```
github.com/umbel/pilosa/*
```
This follows standard Go convention and makes the project "go gettable".
The README was also updated to reflect the changes.
