Renamed hudson.plugins.perforce.security to hudson.plugins.perforce.credentials

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Refactoring: Renamed the P4CredentialsProvider field in order to follow the common PerforceSCM naming approach

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Improved Javadoc documentation

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Added getTitleName() method to P4CredentialsProviderDescriptor

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Refactoring: fixed messages naming

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>