[feature] Additional debugging macro -

Usage summary:
* `xi_debug_logger("Simple message");`
* `xi_debug_printf("[%d] = {%s}", raw->size, raw->data);`
* `xi_debug_logger("Hello, %s!", name);`
[refact] Use new `xi_debug_format()` macro
[feature][debug] Improve printf mactors

* add function name in the output
* add handy `xi_debug_function_entered()`
[build][refact] Overide constants with `xi_user_config.h`
[build][fix] Ensure `XI_DEBUG_*` consts are respected
Merge pull request #68 from xively/few_debug_and_user_config_improvemnts

Improvements to debug macros and user config integration in makefiles