Added support for NotFoundRoute, Redirect, Pagination, Tags

[gesposito]

Fixes #33.

This is a preliminary revision, looking for code review, it's a litte verbose.

v0.13.x API

It adds
<NotFoundRoute handler={_404}/> for root _template.

<Route path="/tag/:tagId" component={_tag}/> for root _template.

<Route path="/page/:pageId" component={_pagination}/> for root _template and every child _template.

<Redirect from="some/where" to="somewhere/else" /> for every page that has the redirect metadata.

Inside _404, _tag, _pagination we can have appropriates Component handlers for those Routes.
Since they should be optional and the missing _404, _tag, _pagination templates could crash gatsby I added the optional configurations (booleans):
notFound
pagination
tags
to enable them (is there a better way?).

Notes

Since we could #32 I can wait for the branch and make the final PR there.

[KyleAMathews]

I don't think 404/tag/category/pagination/etc. belong in Gatsby core as there's dozens of possible extensions like this and I don't want there to be special syntax for each one. I'm a big fan of having a small core with a thoughtful lifecycle/plugin architecture that makes it trivial to add extensions like easy tagging/pagination support.

I have a page of notes on requirements for a plugin system. Let me find some time the next few days to get that posted.

That being said, until there is a plugin system, adding 404 support now would be great. Any real site needs that (like mine). If you want to close this PR and add a new one with the 404 support, I'll merge that right away and once a plugin system lands, we'll migrate 404 support into a gatsby-plugin-404.

Thoughts?