Add MCNP/Serpent plugins and improve documentation

[paulromano]

This PR adds two new plugins: PluginMCNP and PluginSerpent. I've also done an overhaul of our documentation to reflect some of the recent changes. The documentation is now broken into:

1. Getting Started -- this section is new and gives a short overview of watts with an example using MCNP
2. Detailed Usage -- this goes into more detail on each of the major classes (Parameters, Plugin, Results, Database)
3. Available Plugins -- this talks about functionality specific to each of the individual plugins, as opposed to the more general capabilities discussed in "Detailed Usage"

The documentation also features a snazzy landing page:

[nstauff]

Thanks @paulromano - would it be possible to add some examples using MCNP and SERPENT? Once available, I will try these plugins out before accepting the PR.

[paulromano]

@nstauff I've made some updates on this branch. First, I expanded the user's guide description of PluginSerpent to mention serpentTools. I've also added an MCNP example of doing a criticality search with a templated input file. Let me know what you think!

[nstauff]

Thanks @paulromano - this looks great - very neat example for MCNP. We will need to develop one for SERPENT, but I am OK proceeding with this PR now.