[BUGFIX] Forbid array references with leading zeros

[angela-tarantula]

Fixes #63

Use re.fullmatch instead of re.match to forbid indices with leading zeros in order to be RFC6901-compliant.

Before:

>>> from jsonpointer import JsonPointer
>>> JsonPointer("/array/01").resolve({"array": [1,2,3]})
2

After:

>>> from jsonpointer import JsonPointer
>>> JsonPointer("/array/01").resolve({"array": [1,2,3]})
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    JsonPointer("/array/01").resolve({"array": [1,2,3]})
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
  File "/python-json-pointer/jsonpointer.py", line 190, in resolve
    doc = self.walk(doc, part)
  File "/python-json-pointer/jsonpointer.py", line 255, in walk
    part = JsonPointer.get_part(doc, part)
  File "/python-json-pointer/jsonpointer.py", line 234, in get_part
    raise JsonPointerException("'%s' is not a valid sequence index" % part)
jsonpointer.JsonPointerException: '01' is not a valid sequence index