Switch to the Homebrew version of vim

i.e. the one from the macvim package (that comes with ruby/python
support compiled in and includes proper yank to copy register handling,
etc)

For some reason a simple symlink didn't play nicely with the
colorscheme. This seemed easier than figuring out why
Replace Command-T and NERDTree with Unite.vim
Swap tag-list-plus for taglist