Friends support

This introduces two classes:

• ChainSlice is like TChain except it only returns a subset of entries (similar to a slice in Python, [begin:end])
• SlicedFriendElement allows adding ChainSlice to trees' list of friends

A test is currently provided for ChainSlice. The examples test was adapted to compare the results with and without friends.

A demo showing usage is implemented in example02. Changes to darwin.h and Looper.h were needed to facilitate usage.

• Add tests for SlicedFriendElement
• Remove the initial T, move to a Darwin namespace, and reset TOffsetFriendElement serialization version to 1
• Adjust Looper and darwin.h
• Clean up the examples tests (currently they clutter the root command line history)
• Clean up the build system

Cleaning up the build system uncovered #22 again. So,

Closes #22.