update license under which we are using CASSIS

fixes #8
Merge pull request #7 from pfefferle/master

added support for some more post-formats
minor cleanup like line length, spacing, phpdoc
switch to class architecture

hopefully no one was calling those hum_* functions directly.  Not
likely, since all of the interesting integration points had formal
hooks, which this commit doesn't change.
remove debug statement
add support for legacy short url schemes

Allow hum to support the short URL schemes of other plugins or shortlink
generators.  Initially, this just adds support for Friendly Twitter
Links which uses shortlinks of the form /{id} where {id} is the base10
or base32 post ID.  A new 'legacy_hum_id' filter is added to support
additional schemes.  This filter expects a post ID to be returned, which
means this legacy support will only work for WordPress-hosted content.

Resolution of legacy shortlinks happens only after WordPress and all
plugins have failed to resolve the requested URL.  Contrast with
standard Hum shortlinks, which are resolved very early in the process.

Fixes #6