twitter-backup: use fetch_entire_timeline() for incremental download

For some weird reason, the "since" fetch returns 200 *latest* tweets,
not the oldest ones, causing holes if more than 200 tweets have been
posted since the last backup.

With limit 200:

    Loading 4192 tweets from /home/grawity/Backup/tweets.json
    Downloading tweets since 471634183409135617 to latest
    Downloading 200 tweets since 471634183409135617
    Received 199 tweets.
      newest: 488270662474416128
      oldest: 472340222907518977
    Downloading 200 tweets since 488270662474416128
    Received 0 tweets.
    Dumping 4391 tweets to /home/grawity/Backup/tweets.json

With limit 50:

    Loading 4192 tweets from /home/grawity/Backup/tweets.json
    Downloading tweets since 471634183409135617 to latest
    Downloading 50 tweets since 471634183409135617
    Received 50 tweets.
      newest: 488270662474416128
      oldest: 483513810415005697
    Downloading 50 tweets since 488270662474416128
    Received 0 tweets.
    Dumping 4242 tweets to /home/grawity/Backup/tweets.json

Meanwhile, the full-timeline backup downloads everything:

    Loading 4192 tweets from /home/grawity/Backup/tweets.json
    Downloading entire timeline (latest to oldest)
    Downloading 200 tweets
    Received 199 tweets.
      newest: 488270662474416128
      oldest: 472340222907518977
      unseen: 199
    Downloading 200 tweets up to 472340222907518977
    Received 199 tweets.
      newest: 472340222907518977
      oldest: 458993537036873728
      unseen: 12
    Downloading 200 tweets up to 458993537036873728
    Received 200 tweets.
      newest: 458993537036873728
      oldest: 445182186417684480
      unseen: 0
    Dumping 4403 tweets to /home/grawity/Backup/tweets.json
twitter-backup: remove fetch_tweets_since()
Lib.pm: remove the fmt_prefix for _info()

There needs to be a message level between _debug and _log, after all.
twitter-backup: use _info()
Lib.pm: _fmsg() should go to stdout
Lib.pm: allow specifying exit code in _die()
Lib.pm: skip the correct number of frames
gnome-inhibit: use -ll/-tt for extended output
gnome-inhibit: import Lib.pm for logging
gnome-inhibit: no need to manually remove inhibitors
Lib.pm: add _exit() that checks $::errors