Issue #127 The directory name for content should be configurable.

[christianhujer]

Changes:

• New class EpubWriterConfiguration to change the configuration of an EpubWriter.
• Two new constructors for EpubWriter to optionally allow using EpubWriterConfiguration.
• Unit Test EpubWriterConfigurabilityTest with
  • a regression test to ensure the change does not cause any regression to the existing behavior,
  • a test to ensure that the directory name for content is indeed configurable.