Archive key ciphering was added.

hudson.util.Secret class is used to cipher the key. Seems like it's a common practice to use that class to secure passwords in Jenkins.
Some refactoring was done.

Keychain lock timeout was removed (just in case, after it's unlocked, we execute `securoty set-keychain-settings -u $KEYCHAIN_NAME` to clear possible lock timeout).
launcher.launch()... calls were refactored. xcodebuildPath, securityPath etc. variables were renamed to xcodebuild, security etc. for better readability.
Some private variables were made final as they're supposed to be set in constructors.
Merge branch 'develop' of https://github.com/sharpland/iosbuilder-plugin into develop