fixed compatibility with OS version prior to 10.9
- added compile-time guard macros
Adding social_media_url to podspec
Defaulting expiration of tokens to distant future /cc @irlabs
Merge pull request #73 from ChungwaSoft/ChungwaFix

Runtime check for available symbol
re-added the bridging cast to kSecAttrAccessibleWhenUnlocked
Merge branch 'ghmaster' into handle-nil-response-object

* ghmaster:
  Defaulting expiration of tokens to distant future /cc @irlabs
  Adding social_media_url to podspec
  fixed compatibility with OS version prior to 10.9
Merge branch 'reinsert-bridging-cast' into handle-nil-response-object

* reinsert-bridging-cast:
  re-added the bridging cast to kSecAttrAccessibleWhenUnlocked