added logtargets field in MCMCChain

In response to #48 

I added a `logtargets` field to the MCMCChain type and adapted SerialMC, SeqMC, SerialTempMC.
`runtests.jl` passes, so does travis. (note : I have not added tests specific to this field, I just checked them on a few runs.)

@scidom if it is ok with you, i'll merge it in master and tag MCMC at 0.0.6.