Improved doc styling

Here's a preliminary attempt at some improved doc styling. Note that this is **not** ready to merge in directly, as the CSS is not inlined. I've left it like this so people can tweak/modify if they so desire before merging. I'll rebase/squash for the final PR.

Quick summary of changes:

1. Updated stylesheet based on https://bitbucket.org/cskeeters/rst2html-style/ (also uses some styles from http://getskeleton.com/)
2. Lightly modified the HTML generated in nimdoc.cfg to better take advance of above CSS changes
3. I wouldn't say it's *mobile-friendly*, but it's moderately dynamic at least
4. URL source links should be re-enabled (this should be tested across an entire project, but works for me)
5. Headers in documentation now link to themselves, vs. awkwardly moving you back to the TOC
6. No major changes to `rstgen.nim` (except 5), so downstream dependencies should be unaffected


A few comparisons:
* [Old manual.html](http://nim-lang.org/manual.html) vs. [New manual.html](http://a.pomf.se/opilys.html)
* [Old os.html](http://nim-lang.org/os.html) vs. [New os.html](http://a.pomf.se/ddhwlu.html)
* [Old tutorial 1](http://nim-lang.org/tut1.html) and [2](http://nim-lang.org/tut2.html) vs. [New 1](http://a.pomf.se/jcsrmi.html) and [2](http://a.pomf.se/hefatu.html)


Would love to use this PR to get thoughts/feedback/suggestions/tweaks. And then hopefully we can arrive at something everyone likes for v1.0 documentation.