Achieve python 3.x compatibility + resolve TypeError of 'safe' casting from np.int64 types

[dansuh17]

Mainly detecting ImportError and assigning replaceable function existing in python 3.x libraries.

try:
    from itertools import izip
except ImportError:  # python 3.x
    izip = zip
    xrange = range

Done in response to #3 .

---

Also fixes TypeError of message:

Traceback (most recent call last):
  File "test.py", line 12, in <module>
    db.query(fp_q)
  File "/Users/user/qfp/qfp/db.py", line 151, in query
    fp.match_candidates = self._find_match_candidates(fp)
  File "/Users/user/qfp/qfp/db.py", line 172, in _find_match_candidates
    self._filter_candidates(conn, c, qQuad, filtered)
  File "/Users/user/qfp/qfp/db.py", line 206, in _filter_candidates
    if not 1 / (1 + e) <= qQuad.A.y / cQuad.A.y <= 1 / (1 - e):
TypeError: ufunc 'true_divide' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''

Which happens if data are INSERTed while having type np.int64 or other numerical numpy types.
In order to store the actual numerical value, call .item() to the np-typed variables reference.