add an essential build profile to make build more approachable

Newcomers are struggling to build Arquillian for the first time because the default profile steps into all the container modules. This results in massive downloads (JBoss AS embedded, GlassFish embedded, etc), slow build processes and frequent failures. A newcomer to Arquillian shouldn't have to go through this whole parade. They really just want to build the essential bits of Arquillian, then hand pick the container module they want to study or hack.

To cut down on the default build, we should introduce a new profile in the aggregator pom that is activated by default. (Hudson can be set to run with -Pall)

<profile>
<id>essential</id>
<activation>
<activeByDefault>true</activeByDefault>
</activation>
<modules>
<module>build</module>
<module>api</module>
<module>spi</module>
<module>impl-base</module>
<module>junit</module>
<module>testng</module>
<module>testenrichers</module>
<module>protocols</module>
</modules>
</profile>

(I should note that Thomas already did a similar thing to isolate the Arquillian essentials to work on OSGi, but that build profile could be replaced with this one, give or take a module or two).