lib: add `Engine.Measurements.get_all`
lib: add sub-command `inspect`

The command for now the command can only display the measurements in the
DB:

    ketrew inspect all measurements [-e]
lib: add unique i to HTTP request measurements
lib: display `http-request durations` measurement
lib: improve display in `inspect` subcommand
lib: measure more HTTP stats

Now we keep track of HTTP responses and most importantly their size in
bytes.
lib: make `inspect http-request` more complete
lib: add option `--csv` to `inspect`

This also improves the escaping to TSV output.
lib: add option `--since` to `inspect`
lib: update to latest Conduit/Cohttp

Fixes #31.
lib: fix whitespace & comments (PR #33)
Merge PR #33