Mint2 local.properties for local configuration

In order to avoid mint.properties cluttering of test configuration properties, I modified the Config class to support an additional properties file called local.properties. Properties in local.properties have higher priority than custom.properties (which have higher priority than mint.properties). They should not be submitted to the SVN repository and they should be used for local tests during development. They are not required for a Mint2 installation and they will be ignored if missing or empty.