Added ie_core_read_network_from_memory to the C API ie_bridge. (#674)

Added ie_core_read_network_from_memory to the C API ie_bridge. (#674)

* * Added ie_core_read_network_from_memory to the C ie_bridge.

* Added size argument for xml_content, fixed const correctness of the weight_blob, fixed unit test

* * Removed debug message

* Changed variables names from model_xxx to weights_xxx to be more consistent with the argument name of the tested function.

* Added a description for xml_content_size in ie_core_read_network_from_memory.

* * xml_content is now passed as uint8_t
* reading function factorized in the unit-test