Fix TypeError at /services/timing/record

Got TypeError when using with boomerang. It turns out that params are not coerced properly before feeding to _process_summaries function.
```
TypeError at /services/timing/record
unsupported operand type(s) for -: 'unicode' and 'long'
Request Method:	GET
Request URL:	http://127.0.0.1:8000/services/timing/record?client=boomerang&rt.start=navigation&rt.tstart=1395546368776&rt.bstart=1395546372614&rt.end=1395546375459&t_resp=1106&t_page=5577&t_done=6683&r=&t_other=boomerang%7C41%2Cboomr_fb%7C3838%2Ct_domloaded%7C4208&nt_red_cnt=0&nt_nav_type=0&nt_nav_st=1395546368776&nt_red_st=0&nt_red_end=0&nt_fet_st=1395546368927&nt_dns_st=1395546368927&nt_dns_end=1395546368927&nt_con_st=1395546368927&nt_con_end=1395546368927&nt_req_st=1395546368947&nt_res_st=1395546369882&nt_res_end=1395546370000&nt_domloading=1395546370022&nt_domint=1395546372839&nt_domcontloaded_st=1395546372839&nt_domcontloaded_end=1395546372984&nt_domcomp=1395546375371&nt_load_st=1395546375371&nt_load_end=1395546375448&nt_unload_st=0&nt_unload_end=0&nt_spdy=0&nt_first_paint=1395546371.725609&v=0.9&u=http%3A%2F%2F127.0.0.1%3A8000%2Fshanghai%2F
Django Version:	1.4
Exception Type:	TypeError
Exception Value:	
unsupported operand type(s) for -: 'unicode' and 'long'
Exception Location:	/home/tom/.virtualenvs/cw/local/lib/python2.7/site-packages/django_statsd/views.py in _process_summaries, line 65
Python Executable:	/home/tom/.virtualenvs/cw/bin/python
Python Version:	2.7.4
```