Fix identity function

convert time.mktime floats to int for start and end time values

currently attempts to use the identify() function crash with:
```
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/django/core/handlers/base.py", line 111, in get_response
    response = callback(request, *callback_args, **callback_kwargs)
  File "/opt/graphite/webapp/graphite/render/views.py", line 110, in renderView
    seriesList = evaluateTarget(requestContext, target)
  File "/opt/graphite/webapp/graphite/render/evaluator.py", line 10, in evaluateTarget
    result = evaluateTokens(requestContext, tokens)
  File "/opt/graphite/webapp/graphite/render/evaluator.py", line 21, in evaluateTokens
    return evaluateTokens(requestContext, tokens.expression)
  File "/opt/graphite/webapp/graphite/render/evaluator.py", line 29, in evaluateTokens
    return func(requestContext, *args)
  File "/opt/graphite/webapp/graphite/render/functions.py", line 2176, in identity
    values = range(start, end, step)
TypeError: range() integer end argument expected, got float.
```
(technically this stacktrace is from 0.9.x branch.  but offending code is unchanged in master)