[MRG] Rewrite tree2array

[ndawe]

• Speed improvement for trees with many branches: Don't call TTree::FindBranch for each branch since that results in O(n^2). The new algorithm runs in linear time with respect to the number of branches.
• Support for multidimensional variable-length and fixed-length arrays. This resolves Failure when reading complex root file #192.
• If a single branch is requested as a string (not a list of one string), then return a simple 1D array.

Comparing with #193, that particular tree with many branches is now converted twice as fast:

Before:

>>> min(timeit.Timer("tree2array(tree, branches=branches)", setup=setup).repeat(10, 1))
0.362231016159

After:

>>> min(timeit.Timer("tree2array(tree, branches=branches)", setup=setup).repeat(10, 1))
0.189702987671