Pyvolve

Updated to force a numpy random seed reset for each call to Evolver.

Addresses a bug in v0.8.3 where newick tree parser threw an error when encountering a user-labeled root.

Fixed a bug where `scale_tree` in the newick.py module was not applied for user-named internal nodes.
Additionally removed underscore from default internal node names (now internalNodeX instead of internal_nodeX) because underscore can occasionally prevent proper parsing due to model flag interference.

Two primary updates:
1. Custom models can now be provided with custom state frequencies. To provide custom state frequencies, the provided matrix must be symmetric. Under any circumstance, the rows of the matrix should sum to 0 (whether frequencies are provided or not!)
1. Codon models (GY, MG) can now be scaled such that branch lengths indicate the mean number of neutral substitutions per unit time. To invoke, include the argument: `neutral_scaling=True` when defining a Model object, for example:
m = Model("GY", {"omega":0.4}, neutral_scaling=True.

Fixed small typo and updated README.

Resolved Python3 incompatibility issue. Pyvolve is now fully functional for both Python2 and Python3.