FIX: Remove istream Usage in Dirtab Implementation

This removes the `std::istream` file/string reading and replaces it by `ReadLineFile()` and `ReadLineMem()` from `util.h`.

**Note:** This is an amendment to the following Pull Requests:
* [Feature: Pathspec classes](https://github.com/cvmfs/cvmfs/pull/448)
* [Feature: Dirtab Wrapper Class](https://github.com/cvmfs/cvmfs/pull/449)
* [Feature: Simple Catalog Manager](https://github.com/cvmfs/cvmfs/pull/450)
* [Feature: cvmfs_swissknife dirtab](https://github.com/cvmfs/cvmfs/pull/451)
* [Feature: Integrate `cvmfs_swissknife dirtab` into `cvmfs_server`](https://github.com/cvmfs/cvmfs/pull/452)