Removed redundant compiler configuration.

The same configuration is in the master pom.
(Verified by diffing the effective POM as presented by Eclipse m2e.)
Set Java version from property instead of by plugin configuration.

This makes it somewhat easier to override the Java version from
child POMs.
Also, it does not nail down the maven-compiler-plugin version
anymore. Sure enough, this bumped the version from 2.3.1 to 2.3.2
(2.3.2 fixes http://jira.codehaus.org/browse/MCOMPILER-109).
Merge pull request #79 from toolforger/maven-cleanup

Maven cleanup