Gmos/instrument setup

Fixed #11 with this PR. 

Example:

```python
In [2]: tmp = proj.session.query(base.FITSFile).filter(base.FITSFile.fname.startswith('N')).all()[0]

In [3]: tmp
Out[3]: <geminiutil.base.gemini_alchemy.FITSFile at 0x5a5ff90>

In [4]: test2 = gmos.GMOSMOSInstrumentSetup.from_fits_object(tmp)

In [5]: test2
Out[5]: <GMOS MOS Instrument Setup Filter1 open Filter2 open Grating B600+_G5307 Tilt 56.13 central wave=466.00 nm>

In [6]: test2.detectors
Out[6]: 
[<detector id=19 ccdname=e2v DD ML2AR CCD42-90-1-F43 10031-23-05 xbin=4 ybin=4 gain=2.04>,
 <detector id=20 ccdname=e2v DD ML2AR CCD42-90-1-F43 10031-23-05 xbin=4 ybin=4 gain=2.14>,
 <detector id=21 ccdname=e2v DD ML2AR CCD42-90-1-F43 10031-01-03 xbin=4 ybin=4 gain=2.32>,
 <detector id=22 ccdname=e2v DD ML2AR CCD42-90-1-F43 10031-01-03 xbin=4 ybin=4 gain=2.22>,
 <detector id=23 ccdname=e2v DD ML2AR CCD42-90-1-F43 10031-18-04 xbin=4 ybin=4 gain=2.19>,
 <detector id=24 ccdname=e2v DD ML2AR CCD42-90-1-F43 10031-18-04 xbin=4 ybin=4 gain=2.18>]
```