Optimize BetterChain::GetEntry to mimic the behavior of TTreeFormula.

At present, BetterChain::GetEntry uses TChain::GetEntry, which calls
down to TTree::GetEntry.  Unfortunately, TTree::GetEntry iterates over
ALL branches and calls TBranch::GetEntry, and while TBranch::GetEntry is
a no-op if the branch is not active, the iteration incurs considerable
cost.  This commit modifies BetterChain::GetEntry to manually iterate
over only active branches and call TBranch::GetEntry.  This mimics the
behavior of TTreeFormula::EvalInstance, which only loads required
branches on-demand.
Merge pull request #162 from havoc-io/fastload

Optimize BetterChain::GetEntry to mimic the behavior of TTreeFormula.