Support old version git (1.7.1)
Use tr intedad of sed to trim
Raise error in check_error()
Don't use gitstatus-fast.py if conflicts occur
New option (ZSH_VCS_PROMPT_HIDE_COUNT) to hide count
Print unmerged commits count
Logging error and notify the user based on its level

Lv.1: Output error to a file.
Lv.2: Output error to stderr.
O/W : Suppress erorr.
Update README.md
Refactoring
Provide the way to measure running time

Add ZSH_VCS_PROMPT_DEBUG_THRESHOLD_MICRO_SEC option