Upgrade to lepton introduces warnings in VS2013

[chrisdembia]

PR #19 introduced warnings in VS2013. @aseth1 said he sees warnings like:

warning C4267: '=' : conversion from 'size_t' to 'int', possible loss of data, e.g:
void ExpressionProgram::buildProgram(const ExpressionTreeNode& node) { for (int i = node.getChildren().size()-1; i >= 0; i--) ...}

I see two resolutions:

1. Patch github.com/simtk/openmm to get rid of this warning @peatsman, do you build with VS2013? If so, do you see such warnings?
2. Fix the warning ourselves in opensim. I think we'd do this by changing int i to size_t i. We might need to include stdint to get this to work on VS2010.

Looking for suggestions.

[sherm1]

I think you meant @peastman, Chris. Peter, please see above message.