add support for jdk 6 javac -processorpath parameter

add support for annotation processing javac option -processorpath
cf. http://download.oracle.com/javase/6/docs/technotes/tools/windows/javac.html#processing

while not supported, annotation processor classes have to be supplied via compile classpath.

annotation processor dependencies (e.g. database, xml processing, ...) might not be wanted as project dependencies or event be in conflict with them.

as a workaround I have attached new ProcCompilerMojo, where I tried to implement processorpath support
1) via a new configuration option: dependency resolution is done with bits of code I copied from other maven classes like ProjectBuilder, ...

<configuration>
	<proc>only</proc>
	<processorpath>
		<dependency>
			<groupId>org.slf4j</groupId>
			<artifactId>slf4j-jdk14</artifactId>
			<version>1.5.6</version>
		</dependency>
		<dependency>
			<groupId>com.example</groupId>
			<artifactId>annoproc</artifactId>
			<version>1.0.0</version>
		</dependency>
	</processorpath>
</configuration>

2) (unused) via plugin dependencies: I basically make use of ProcCompilerMojo classloader urls. This variant is quite "dirty". it adds unnecessary maven plugin jars to annotation processor classpath and unnecessary annotation processor jars to compiler-plugin classpath