add profile.
add shell/path.
add zshenv.
rearrange scripts sourcing between RC files.
make shell/rvm loadable by multiple RC files.
rename dot_bin to bin.
use functions in shell/path to add paths to $PATH.
shell/path: fix logic.

if any duplicate is found, remove it, and re-append/insert it.
shell/path: add new function -- mdx_uniq_path().
zshrc: add call to mdx_uniq_path().
zshenv: source shell/path.
profile: source/bin instead of dot_bin.