Check for UnknownFrame when converting ID3v2 frames (#31)

Looking at the ID alone to determine the frame class is not enough. E.g.
for deprecated frames like TDAT, TagLib keeps the frame ID but returns
it as an UnknownFrame.

This change fixes the handling of such frames and adds a note to the
documentation of frame_list.
Merge pull request #32 from robinst/unknown_frame

Check for UnknownFrame when converting ID3v2 frames (#31)
Associate filesystem encoding to filename strings

In Ruby 1.9, strings containing paths (such as those returned by Dir[])
have the filesystem encoding associated to them. This commit makes sure
the same holds true for FileName strings returned by TagLib.
Merge pull request #30 from robinst/filename_encoding

Associate filesystem encoding to filename strings
Regenerate SWIG wrappers using swig 2.0.10
Merge pull request #35 from robinst/swig_2.0.10

Regenerate SWIG wrappers using swig 2.0.10
Document some gotchas of mutable attributes

For convenience taglib-ruby returns Strings and Arrays in many places. A
user might expect that if they modify such an attribute in-place, the
modification is propagated to the underlying C++ object, which is not
the case. This commit adds documentation for this behaviour.
Merge pull request #39 from robinst/doc_mutable_attributes

Document some gotchas of mutable attributes
Recompile wrappers using SWIG 2.0.11
Merge pull request #40 from robinst/swig_2_0_11

Recompile wrappers using SWIG 2.0.11
Document TagLib::ID3v2::TextIdentificationFrame construction (#46)
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
Update build instructions in README (see #54)
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)