Check the root package for sniffs to install

[Potherca]

Problem/Motivation

As described in #19, the installer does not look in the root package. As it is completely viable for root packages to contain a code sniff, the installer should also check the root package.

Expected behaviour / Actual behaviour / Steps to reproduce

Not applicable.

Proposed changes

A separate step needs to be implemented that checks the root package to see if it contains any sniffs that (also) need installing.

[frenck]

Added this feature in PR #25 and is waiting code review.

[frenck]

Merged into the master branch and will in the next release.