first simple implementation of the Spectrum1D class
added a couple more from_methods to instantiate from tables and files.
updated preamble and improved some minor documentation
Added documentation for interpolate method and introduced code for handling errors, masks and flags during interpolation
separated slice method into explicit slice_index, slice_dispersion methods and added documentation. this assumes NDData will have a slice-like object to perform slices on all the data objects (incl. error, masks, etc)
from_dispflux doesn't exist, I think this was meant to be from_array. overwritten to from_array for consistency in either case.
switched SciPy check into interpolate method (which is the only method so far that needs it), following astropy/nddata/convolution/make_kernel.py precedent
minor addition
updated slice_index documentation
Removed backticks from data references in exception messages
Improved documentation for Spectrum1D class and initialisation inputs more closely reflect NDData
Merge pull request #1 from andycasey/master

Improved documentation and some consistency impovements
merged with current master
adjusted slightly to reflect the changes to NDData
Merge pull request #20 from wkerzendorf/master

Cleaning of Spectrum1D and some documents