feat($cordovaKeychain): add keychain support

- documentation coming next
- closes #28
Merge remote-tracking branch 'upstream/master' into new-plugin-keychain

Conflicts:
	src/plugins/module.js
refactor($cordovaKeychain): add promises
Merge pull request #198 from mrzmyr/new-plugin-keychain

feat($cordovaKeychain): add keychain support