Use OS-specific directory separator instead of hard-coded forward slash

Fix issue with cURL on Windows machines when the local CA cert file 
is used. For some reason cURL refuses to use the CA cert bundle if 
forward-slashes are present in its path name.
Added validation and test for non-int error_code defaulting to 0.
Merge pull request #78 from alixandru/master

Use OS-specific directory separator instead of hard-coded forward slash
Merge pull request #106 from gfosco/error_code

Added validation and test for non-int error_code defaulting to 0.
Merge branch 'master' of github.com:facebook/facebook-php-sdk
Merge branch 'master' of github.com:philsturgeon/facebook-php-sdk