eliminated unnecessary cd and failing substitution

Changing the working directory in a sub-subshell
does not change the working directory of the executing
shell.

The substitution was broken for me on _all_ my machines,
so I started looking into the business.
Adding a fix for the DISABLE_UNTRACKED_FILES_DIRTY option.
Add web-search plugin.

This plugin adds google, bing and yahoo commands to launch the default
web browser to do web search:

e.g.

    google oh-my-zsh
    bing what is zsh
Fixed recursion. Git not needed for it to work.

Avoid infinite `cd` loops under certain conditions. Try getting `.venv` from the current directory (not necessarily always using git)
Merge pull request #1839 from Kronuz/patch-1

Fixed recursion. Git not needed for it to work.
Merge pull request #1837 from xuhdev/web-search

Add web-search plugin.
Merge branch 'master' of https://github.com/robbyrussell/oh-my-zsh
Merge pull request #1775 from marcusmueller/fixupgradetool

fix for failing substition & eliminated unnecessary cd
Merge pull request #1834 from croach/disable_untracked_files_dirty_fix

Adding a fix for the DISABLE_UNTRACKED_FILES_DIRTY option.
Merge branch 'master' of https://github.com/robbyrussell/oh-my-zsh

Conflicts:
	tools/upgrade.sh
fix the open command in linux using xdg-open
Merge pull request #1849 from lemieux/master

Fix for Jira plugin on Linux
Update README.textile

Add upgrade command to docs. I canceled the prompt once and went hunting for this so I could upgrade.
Merge pull request #1870 from mfrobben/patch-1

Update README.textile
Merge branch 'master' of https://github.com/robbyrussell/oh-my-zsh