khash.h updated to 0.2.6 (our version)

* The capacity is a power of 2. This seems to dramatically improve the
  speed for simple keys. Thank Zilong Tan for the suggestion. Reference:

   - http://code.google.com/p/ulib/
   - http://nothings.org/computer/judy/

* Added Wang's integer hash function (not used by default)
doc: document members of PNVtable. better khash desc