Finished importClass() and importFromClass() NAMESPACE directives

Makes it easier to include JVM classes and methods in R packages
Added some optimized code for dist() to stats package
Miscellanious optimizations

- c() now defers long combinations n > 2000
- MultithreadedVectorPipeliner turned on by
  default for CLI and GUI
- is.finite(), is.infinite() marked as deferrable
Removed debugging messages
[maven-release-plugin] prepare release parent-0.7.0-RC6
[maven-release-plugin] prepare for next development iteration
Merge branch 'master' into wip-optimize

Conflicts:
	core/src/main/java/org/renjin/compiler/pipeline/VectorPipeliner.java
	core/src/main/java/org/renjin/compiler/pipeline/accessor/Accessors.java
	core/src/main/java/org/renjin/primitives/vector/CombinedDoubleVector.java
	packages/stats/src/main/java/org/renjin/stats/dist/DistanceMatrix.java
	packages/stats/src/main/java/org/renjin/stats/dist/Euclidean1dDistanceTriangle.java
Made IR tree fully mutable

Checking SSA transform