Support for custom IO classes merged to master

[Lukáš Lalinský]

Many people were interested in abstracting out the file operations.
The master branch on GitHub now makes it possible to create a new
class based on TagLib::IOStream and have TagLib::File use it instead
of its own file-based implementation. I'd love if people who wanted
this functionality had a look and let me know if it works for them or
they would like to have something changed. Thanks.

Lukas