Add Pathogen for easier vim runtime file / plugin management

https://github.com/tpope/vim-pathogen
Add vim-octopress for nicer syntax hilighting of Octopress files
Pathogen .vimrc comment made somewhat more useful with a URL
Set vim filetype to octopress for files in Octopress-y path

(Future me: if I'd been able to think of a better way, I would have used
it)