Split generated files in two

[dmfs]

With this commit the plugin generates two files:

• one that contains all the constant String values and
• one that contains all the *UID values.

This way, the generated Strings can be used in non-OpenHab environments reducing redundancy and risk of typing errors.

To support this, the plugin now expects two distinct class name properties called esh-constants.openhabClassName and esh-constants.stringsClassName

In addtion the following changes were made to improve maintainability:

• switch from generating Java code with Freemarker to creating it with Javapoet
• switch from testing generated sources by comparing the text character-wise to comparing the AST using Google's compile-testing lib

[dmfs]

I forgot to update the README.md. Should be ok now.