Use Hash Abstraction API in File Processing Classes

This replaces the direct calls to openssl's SHA1 API by the abstraction layer provided by cvmfs. In order to make this work, I needed to slightly extend the cvmfs API.

**Note:** IMHO this abstraction layer needs a full rewrite!

**Note:** this contains [Refactoring of the Asynchronous Reader](https://github.com/cvmfs/cvmfs/pull/246)
**Note:** this contains [Add Unittests for new components](https://github.com/cvmfs/cvmfs/pull/245)
**Note:** this contains [Integrate the TBB based file processing into the Spooler](https://github.com/cvmfs/cvmfs/pull/244)
**Note:** this contains [New File Processing Pipeline based on TBB](https://github.com/cvmfs/cvmfs/pull/243)