Bump to 2.0.3

Version 2.0.3 was already released, so the library should use (at least) this version.
Merge pull request #4 from xhochy/patch-1

Bump to 2.0.3
Fix export macro to not always export all symbols
Disable -Werror
Bump SOVERSION to 2.1
warning
Add artist year before/after dates to Song API
Add test for new API
Tweak/fix tests
added acousticness to AudioSummary and fixed testcase