configuration loading should not manage InputStream, use libs instead

[FedorRomanov]

Configuration::readConfiguration(@Nonnull final String uri) should use time-proven libs to load config remotely/locally instead of managing low-level details. Taking advantage of the libs reduce length of the method and improve readability by hiding low-level details.

1. For loading text file over http/https use apache http client EntityUtils
2. For loading text file from local filesystem, use Java7 API, e.g. new String(Files.readAllBytes(Paths.get(new File(uri))))
3. For loading from classpath, use ClassPathResource in solution above

[rijnb]

Marked as an enhancement. The code presented works and has been tested, but may be improved. Will look at it later.