0.8.0

Changes in this release:

• Added ability to import/export the index, via new import_index and
  export_index methods on the IndexedGzipFile class (#7, #8) - thanks
  @ozars !
• Deprecated SafeIndexedGzipFile - the IndexedGzipFile is now
  thread-safe, and has a read buffer. If you don't want buffering
  or thread-safety, use the _IndexedGzipFile class.
• Fixed bug in seek method - was not working with negative offsets.
• More unit tests, and code coverage of .py and .pyx files.