Grab-bag of cosmetic changes

[kbarbary]

This PR contains no changes in functionality. Rather, these are changes to better conform to Julia conventions and changes to make the code easier to understand. I was able to clear up a few TODOs in the code along the way. The commit messages are a pretty good summary of the changes, but here's a bit more:

Minor:

• remove @doc macro everywhere; not needed on Julia 0.4
• Int64->Int everywhere. More correct (...if we ever ran on 32-bit) and shorter
• use function keyword for multi-line constructors rather than begin ... end block.
• merge benchmark_elbo.jl and benchmark_elbo_with_hessian.jl scripts; they were almost identical.

More significant:

• Write-out ParamSet type definitions rather than using metaprogramming to define them. Only slightly longer and much more understandable.
• Use submodules rather than putting src directory on the Julia load path. This makes it a little more obvious what namespace things are defined in.
  Files containing submodules are in CamelCase.jl whereas files that are just included into other modules are lowercase_with_underscores.jl.

[kbarbary]

What's the deal with forward_diff_model_params()? It doesn't seem to be used anywhere in Celeste, but there's a bunch of tests on it.

[jeff-regier]

Looks good Kyle. We might want to search for dead code some time too. @rgiordan will know about forward_diff_model_params.

[rgiordan]

Thanks for doing all this, @kbarbary ! forward_diff_model_params is only for unit tests -- it converts a ModelParams object containing floats to one containing ForwardDiff GradientNumbers (or whatever), which allows us to text coded derivatives against autodiff derivatives.

[kbarbary]

Should probably move forward_diff_model_params to tests somewhere then, just to make this clear for the future.