updated url to latest powerline-patched fonts

clearing up some confusion in issue#1906
corrected branch character

changed ± to 
theme agnoster: remove trailing space

re-implemented expired pull request https://github.com/robbyrussell/oh-my-zsh/pull/1759
Merge pull request #2479 from mrbfrank/theme-agnoster

theme agnoster: update characters to match latest powerline fonts