Fixed method_missing for backends

`method_missing` should not be implemented as a catch-all solution.
Instead, this version catches only relevant missing public API methods
and delegates the rest to the original implementation.
Strict detection of public backend API methods
Added Errors::ConfigurationError

Used for reporting semantic errors in configuration.
Updated logging facilities

* Added ROCCI_SERVER_LOG_DIR variable (e.g. for configuration from VHs in Apache2)
* Fixed logger initialization
* Enabled tagged logging only for production env
* Enabled colorful logs only for development env
Fixed init of Occi::Log