New parallel installer using +python
- Thanks to @starcraftman

Bug fixes and improvements
- Fix #139 - Use git fetch+merge instead of pull
- Fix #145 - Merge duplicate on-demand loading triggers
- Fix #155 - `cd /d` on Windows
- Fix #159 by temporarily setting &shell to sh
- Use /bin/sh instead of /bin/bash (PlugSnapshot)
- Fix #162 - Do not use --progress option on git 1.7.0 or below
- Fix #154 - issues with paths containing spaces
- Fix #166 - v:version >= 703 || has('patch-7.2.374')
- Do not escape spaces in &rtp
- Remove `git::@` from URI if git 2.3.0 or above is found
- Add `o` key binding for PlugDiff
- Fix #177 - Check Python version >= 2.7
- Fix #174 - Update of tagged plugins
- Interpret the second argument to Plug command as tag option
- Fix #154 - error on windows when plug home contains spaces
