Update CHANGELOG for 0.5.1

Changelog: mention the JSON API change

Release of Mirrorbits 0.4

First release candidate for the 0.4 upcoming release

After more than a year of development, Mirrorbits version 0.3 is here!

Changes:

========

cli:
- Request a scan using a specific protocol (rsync or ftp)
- Print basic download stats (mirrorbits stats <identifier>)

core:
- Improve parse errors in the configuration
- Don't log if logdir in unset
- Fix a minor corner case when the client and server are in the exact same
  location.

This version fixes a possible division by zero during mirror selection.

It is strongly advised to upgrade.

This is the second alpha release of mirrorbits.

Changes:
========

cli:
    - A parse error in the mirror configuration can now be retried
    - Add supports for taking notes / comments on a mirror
    - Add a flag to auto-enable a mirror after a successful scan
    - Add a flag to scan all mirrors at once

core:
    - Improved mirror selection algorithm
    - Fix few corner cases in weight distribution

Optimism is an occupational hazard of programming: feedback is the treament.
    --Kent Beck

First alpha release