Rename log back to logger

The accepted idiom for using this package is to create a new logger object and
reference that for logging output. The library became too cluttered using a
global space as well as object space for all logging functions.
template.go: Cleaner output template
logger.go: Remove package space functions
logger.go: Change *Prefix to *Label
logger.go: Move New()
logger_test: Update tests to reflect changes