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
added failure returning if no internal error created
Merge branch 'experimental-oauth-2-error-response' into working-branch

* experimental-oauth-2-error-response:
  added failure returning if no internal error created
Merge branch 'ghmaster' into use-built-in-basic-auth

* 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 '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 use-built-in-basic-auth

* reinsert-bridging-cast:
  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
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
Merge branch 'Issue66-credential-expiration-fix'

* Issue66-credential-expiration-fix:
  re-added the bridging cast to kSecAttrAccessibleWhenUnlocked
  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 'use-built-in-basic-auth'

* use-built-in-basic-auth:
Merge branch 'handle-nil-response-object'

* handle-nil-response-object:
Merge branch 'master' into working-branch

* master:
  re-added the bridging cast to kSecAttrAccessibleWhenUnlocked
  Defaulting expiration of tokens to distant future /cc @irlabs
  Adding social_media_url to podspec
  fixed compatibility with OS version prior to 10.9