ENH: Speeding up filtering

Apparently the attenuation check can take up a substantial amount (~10-50%) of the time in FIR/FFT filtering:
```
>>> raw.crop(0, 60, copy=False)
<Raw  |  n_channels x n_times : 401 x [60001]>
>>> t = time.time(); raw.filter(None, 55, n_jobs=6); print time.time() - t
24.48
>>> t = time.time(); raw.filter(None, 55, n_jobs=6, skip_check=True); print time.time() - t
12.81
>>> t = time.time(); raw.filter(None, 55, n_jobs=6, filter_length=8192); print time.time() - t
6.68
>>> t = time.time(); raw.filter(None, 55, n_jobs=6, filter_length=8192, skip_check=True); print time.time() - t
5.81
```
This adds an option to skip the check for advanced users who have already considered the filter length tradeoffs.