Levenshtein

* a segfault in setratio() was fixed
* median() handles all empty strings situation more gracefully

* new functions seqratio() and setratio() computing similarity between
string sequences and sets
* Levenshtein optimizations (affects all routines except median())
* all Sequence objects are accepted, not just Lists

* setmedian() finding set median was added
* median() initial overhead for Unicodes was reduced

* ratio() and distance() now accept both Strings and Unicodes
* removed uratio() and udistance()
* Levenshtein.c is now compilable as a C library (with -DNO_PYTHON)
* a median() function finding approximate weighted median of a string
set was added

* Initial release