Import 2.0 Catalogs into a fresh 2.1 Repository

This Pull Request contains the following:
* python tools for catalog management
  * download all catalogs of a given repo
  * open a specified nested catalog for SQL inspection
* extended catalog creation API
  * allows storage of the `catalog hash` in the (read-only) catalog --> simpler bookkeeping
  * refactored database schema creation --> more flexibility
* catalog traversal template allows to keep loaded catalogs in memory
* added `cvmfs_server import` command to import 2.0 catalogs into a fresh 2.1 repo
* refactoring of the `cvmfs_server` script --> more flexibility in repository creation
  * creation of repository scaffold is needed for `mkfs` and `import`
  * general readability improvements (environment sanity checks)
* comparison and assignment operators for the `ShortString` template
* `cvmfs_swissknife migrate` subcommand responsible for converting 2.0 into 2.1 catalogs
* concurrency changes
  * added a simple `Future` template
  * removed precaching feature (clashed with `Future`)
* Testcase to validate the import functionality