Fix canAccessKeychain crash on Mac OS X

Merge pull request #6 from square/federman/fix_mac_canAccessKeychain

Make canAccessKeychain not crash on Mac

Adjusted enum names to use Cocoa conventions

VALMigration*Error -> VALMigrationError*
VALAccessible* -> VALAccessibility*

Inspired by issue #2

Initial Release

A Better Place to Put Your Keys