NUTS implementation

[xukai92]

Before implementing the NUTS sampler I think it's better for us to clear up the following issues:

• Merge ParticleSampler and HMCSampler into Sampler? #174 Merge ParticleSampler and HMCSampler into Sampler?
• Initial step size before heuristically search #183 Initial step size before heuristically search
• Dual Averaging (with default setting suggested by NUTS paper) doesn't work well in transformed space #182 Numerical problem for Dirichelet

Also it may be good to implement the Algorithm 3 in the NUTS paper first before (the one without Dual Averaging), just in purpose of make one part of NUTS work first.