GOES analysis - Temperature and Emission Measure

Hi guys.  I've written some code to calculate GOES temperature and emission measure in sunpy.  This is my first pull request so I hope I've done all this right.  But let me know if not.

I've written four functions in goes.py: temp_em, goes_chianti_tem, goes_get_chianti_temp and goes_get_chianti_em.  The latter three do the same jobs as the procedures of the same name in SSW.  temp_em is a wrapper which allows you to simply input a GOESLightCurve object, then calls the other functions to give you back a new GOESLightCurve object which also includes temperature and emission measure.

I've also created a new module, exceptions.py, which contain a number of small functions which verify the type of various inputs.  These functions are called by the functions I wrote in goes.py.  I'm not sure what the organisational ethos of sunpy is.  So if there's a better place to put these let me know.

In addition, I've created a new directory, instr_files, which contains csv files (and scripts which created them) which are called by the new functions in goes.py.  Once again, I don't know if there's a better place to put these files, so let me know if there is.

Finally, there's another new script, goes_temp.py.  Please ignore this.  It's a temporary file I'm using to develop further GOES analysis tools for sunpy.

Let me know if you have any suggestions for the code or find any errors.  I've also included some simple unit tests for the four new functions in goes.py to help with this.

Cheers!