log, errors: Added lumberjack library for log rolling

The errors and logs can now have log rolling if provided by the user.
The current customisable parameter of it are:
The maximal size of the file before rolling.
The maximal age/time of the file before rolling.
The number of backups to keep.

browse: Initial test for json response in browse.go

Redundant comments in the code removed

middleware: Initial test for replacer

Fixing comment

the old comment might throw the source-reader off, my bad.

browse: Optional JSON output

Complete test coverage for recorder.go of middleware package

Initial test for middleware/middleware.go

Fixes bug introduced in 0ac8bf58 - Also note that setup functions no longer have access to server port. Will need to fix later.

log, errors: Support system log (except Windows)

Specify log file of 'syslog' to output to system log

Also change sha1 to md5 for the directory scans; slightly faster.

See github.com/russross/blackfriday/issues/189

This behavior can still be overridden by bind directive

markdown: generate static sites after links

markdown: Watch for file changes for links. Removed sitegen requirement for link index.

Bug detected by go-fuzz (more fuzzing is required; need larger corpus)