Merging peristent set/map implementation efforts.
Added Maven command to download Google's Java Code Style.

This work is related to issue cwi-swat/rascal#517.
Extending dynamic types tests for IMap and fixing shortcomings of 'fast' implementation.
Fix problem with dynamic type calculation in SetWriter class.
Changing visibility of subclasses and statistics functionality.
Renaming 'CompactNode' to 'CompactMapNode'.
Renaming 'AbstractNode' to 'AbstractMapNode'.
Renaming specialized classes.
Renaming inner static interfaces.
Using the same code generator for specialized set/map nodes. Made their codebases more equal.
Fixing deletion behaviour to keep Trie consistent.

Also fixed a copy and paste bug in PDBPersistentHashSet.
Added a missing break statement in readString in the \U case.
made writer robust against illegal unicode, just in case
Merge branch 'master' of github.com:impulse-org/pdb.values
merged master