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
Merge branch 'ghmaster' into Issue66-credential-expiration-fix

* 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

Conflicts:
	AFOAuth2Client/AFOAuth2Client.m
re-added the bridging cast to kSecAttrAccessibleWhenUnlocked
Merge branch 'reinsert-bridging-cast' into Issue66-credential-expiration-fix

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