#59)

Motivation:

It's currently not possible to use the detected properties to activate profiles.

Modifications:

PR implements afterSessionStart in addition to afterProjectsRead. This makes it possible to use the detected properties to activate profiles.

```
...
<profile>
  <id>Ubuntu</id>
  <activation>
    <property>
      <name>os.detected.release</name>
      <value>ubuntu</value>
    </property>
  </activation>
...
  <!-- Now you can do Ubuntu-specific work -->
...
</profile>
```

Result:

`os-maven-plugin` is now capable of injecting the detected properties even before reading `pom.xml`.