* Cleaned up usage of handles vs. handle pointers
* Store global string for last error thrown
* Removed uhd::device_addr_t handle, added std::vector<std::string> handle
* multi_usrp, multi_usrp_clock, and associated classes accessible through C
* Added Doxygen documentation explaining structure and API
* Simple RX and TX streaming examples
* Unit tests for different parts of C interface and C++ error conversion