Add Syntastic.

I've been using Syntastic for months now, and I'm really happy with having it around when editing Python code.  It works for numerous other file types, if you have the supporting machinery.  Unfortunately, some of the tools can be rather verbose and require lots of customization if you don't want them to be, so I have it set to only allow errors to get through.  Still, it's beneficial.

Currently, I only have active checking turned on for python, vim, and ruby.  We should probably consider others (sh, zsh, html, xml, and yaml), but I thought this was a good place to start.  FWIW, I've been running with them all turned one for months now.  The only one that really frustrates me is the rst checker, since it doesn't recognize the Sphinx extensions.  Other than that, I've been pretty happy.  So we could widen this, if you like.

A little ways back, I added the ability to control the color of the signs and lines in Syntastic (034727a0).  I currently have this in `szakdark.vim`:

```vim
" Syntastic
hi SyntasticErrorSign   guifg=white     guibg=#af0000   ctermfg=white   ctermbg=124
hi SyntasticWarningSign guifg=white     guibg=#aaaa00   ctermfg=white   ctermbg=136
hi SyntasticErrorLine   guibg=#3f0000   ctermbg=52
hi SyntasticWarningLine guibg=#2f2f00   ctermbg=58
```

We may want to consider doing something for others because I find it *extremely* helpful to have the errors and warnings colored.  I can go back and check to see if the colors are defined with `VimEnter` auto command, and default to the above (or something else more suitable), if you like.