Fix UDF regressions. Fixes #29. Add UDF exec tests

* Pulled in a newer `udf.h`

* Impala UDF named structs now pulled out of precompiled module with dummy fn

* UDFs now link a fresh precompiled module into each UDF

* Split C++ code into string implementations and type spec; updated `Makefile` and `setup.py`

* ImpalaContext now cleans up UDFs and UDAs

* Switched UDF modules to absolute imports