Add support for AIFF

- add support for
  - TagLib::RIFF::File abstract class
  - TagLib::RIFF::AIFF::File class
  - TagLib::RIFF::AIFF::Properties class

- added swig code for RIFF and AIFF.
- added test code for reading/writing tags (ID3v2)
    to/from AIFF files, including cover-arts.
- added an AIFF-encoded audio file for tests.
- added documentation.
generate swig wrappers for TagLib 1.7
Allow CXX override during gem installation

TagLib 1.9.1 does not get along with clang++ on OS X 10.9, see
https://github.com/robinst/taglib-ruby/issues/44 . Overriding the C++
compiler used to compile taglib-ruby native extensions seems to solve
these problems.

Poking around in RbConfig feels like a hack but it is the best solution
I can come up with. I looked to Nokogiri for inspiration:

https://github.com/sparklemotion/nokogiri/blob/8f22560ab796e77a9a637c92cd5bf79261f1ad84/ext/nokogiri/extconf.rb#L299
Merge pull request #53 from robinst/change_cxx

Allow CXX override during gem installation
remove tests for TagLib 1.8+ features
remove unnecessary RIFF::File class
add test code for examples shown in the documentation

this is to ensure that examples actually work
(first run test code, then copy/paste to documentation)
add test code for examples shown in the documentation (oops)

this is to ensure that examples actually work
(first run test code, then copy/paste to documentation)

(commit missing file test/aiff_examples_test.rb)
regenerate AIFF wrapper using swig 2.0.11 (#54)
Update build instructions in README (see #54)
fix spelling typo
Merge pull request #52 from tchev/aiff

Add support for AIFF
Ensure swig is installed on Travis build
Merge pull request #56 from robinst/travis-swig

Ensure swig is installed on Travis build
Also build for Ruby 2.1.1 on Travis
Merge pull request #55 from robinst/travis-ruby2.1.1

Also build for Ruby 2.1.1 on Travis
Add note how to force SWIG regeneration (see #54)