possible to disable caching (#8)

by defining the variable ZSH_THEME_GIT_PROMPT_NOCACHE, the caching is disabled, so the prompt is updated at every refresh
Python3 compatibility (#4,#6)
minor fixes to README
fix the examples in README
Readme: explanation of prompt structure
fix git commands working inside .git folder (#5)

some git commands still work inside a .git folder, so the prompt thinks we are in a regular git repo
Only set prompt dir if it has not already been set. Useful for overriding from shell init files.
Add precmd_update_git_vars to update the Git section before prompt renders
more pythonic way to find 'fatal' in string

use 'in' instead of find.
Merge remote-tracking branch 'xaq2892/master'
Revert "Only set prompt dir if it has not already been set. Useful for overriding from shell init files."

This reverts commit 50615c9e2de2e2f5fd890e6bc17f899e6d1cd4a6.