Added API for properties.
Add an example for properties.
Require taglib >= 1.8.
Read properties directly from the File.

Id3v2 tags are now all correctly read. We might want to cleanup in order
to avoid duplicates between code for tags' properties and files'
properties.
Cleaned up, also move set method to File module.
More cleanup.
Merge pull request #1 from savonet/properties

Added API for properties.