Add support for nested keys

[Tomkowski]

Current solution does not allow nesting keys in variable expansion for the ＠Key annnotation.
Let us assume the following scenario:

    @Key("environment")
    @DefaultValue("global")
    String env();

    @Key("environments.${environment}.webdriver.url")
    String webDriverUrl();

    @Key("environments.${environment}.browser")
    @DefaultValue("chrome")
    String usedBrowser();

I want to have a property based on the selected browser property which is already based on the selected environment property. I am trying to access the property in the following ways:

    @Key("environments.${environment}.webdriver.${usedBrowser}.switches")
    String webDriverOptions();

    @Key("environments.${environment}.webdriver.${environments.${environment}.browser}.switches")
    String webDriverOptions();

but neither solution is working.
This pull request adds ability to nest variable expansion inside @Key annotiation.

[Tomkowski]

Interestingly enough, my solution also enables properties to have curly brackets in its name. E.g.: {name} is now a valid property name. I am not sure whether it is a real case scenario or just some corner case.