Switch to using public API PyUnicode_AsUTF8()

Amazingly, Python does not make us responsible for freeing the resulting
string once it has mixed the Unicode down to UTF-8!  Instead, it caches
a pointer to the string on the Unicode object itself, and is careful to
free it when the object goes away.  So, no memory leak!
Merge branch 'master' of github.com:brandon-rhodes/pyephem

Conflicts:
	extensions/_libastro.c