Some small cleanups.

 - Simplify .gitignore
 - Fix warning + errors during build.
 - Rename and move string util method.
 - Introduce GeoServer data object to store API access parameters.
Use GeoServer instance for server parameters in REST reader.
Use slf4j with logback for logging.
Replace url, username and password with GeoServer instance.
More dependency injection.
Upgrade httpclient dependency.
Convert HttpUtils to GeoServerClient.

 - Moved string methods to StringUtils class.
 - Made all other util methods instance methods instead of static.
 - Renamed class to reflect changes.
Reduce console output.