Add Int8RangeType (#401)

This is just like IntRange, but supports greater bit depth.

sqlalchemy_utils/__init__.py

@@ -74,6 +74,7 @@
     instrumented_list,
     InstrumentedList,
     IntRangeType,
+    Int8RangeType,
     IPAddressType,
     JSONType,
     LocaleType,

sqlalchemy_utils/types/__init__.py

@@ -29,6 +29,7 @@
     DateRangeType,
     DateTimeRangeType,
     IntRangeType,
+    Int8RangeType,
     NumericRangeType
 )
 from .scalar_list import ScalarListException, ScalarListType  # noqa

sqlalchemy_utils/types/range.py

@@ -146,6 +146,7 @@ class Event(Base):
 from sqlalchemy.dialects.postgresql import (
     DATERANGE,
     INT4RANGE,
+    INT8RANGE,
     NUMRANGE,
     TSRANGE
 )
@@ -363,6 +364,58 @@ def __init__(self, *args, **kwargs):
         self.interval_class = intervals.IntInterval
 
 
+class Int8RangeType(RangeType):
+    """
+    Int8RangeType provides way for saving ranges of 8-byte integers into
+    database. On PostgreSQL this type maps to native INT8RANGE type while on
+    other drivers this maps to simple string column.
+
+    Example::
+
+
+        from sqlalchemy_utils import IntRangeType
+
+
+        class Event(Base):
+            __tablename__ = 'user'
+            id = sa.Column(sa.Integer, autoincrement=True)
+            name = sa.Column(sa.Unicode(255))
+            estimated_number_of_persons = sa.Column(Int8RangeType)
+
+
+        party = Event(name=u'party')
+
+        # we estimate the party to contain minium of 10 persons and at max
+        # 100 persons
+        party.estimated_number_of_persons = [10, 100]
+
+        print party.estimated_number_of_persons
+        # '10-100'
+
+
+    Int8RangeType returns the values as IntInterval objects. These objects
+    support many arithmetic operators::
+
+
+        meeting = Event(name=u'meeting')
+
+        meeting.estimated_number_of_persons = [20, 40]
+
+        total = (
+            meeting.estimated_number_of_persons +
+            party.estimated_number_of_persons
+        )
+        print total
+        # '30-140'
+    """
+    impl = INT8RANGE
+    comparator_factory = IntRangeComparator
+
+    def __init__(self, *args, **kwargs):
+        super(IntRangeType, self).__init__(*args, **kwargs)
+        self.interval_class = intervals.IntInterval
+
+
 class DateRangeType(RangeType):
     """
     DateRangeType provides way for saving ranges of dates into database. On