Documentation improvements

[matsvanes]

Keep track of the OSL paper requirements:

• Put links in the API for the preproc config, and GLM design, pointing to tutorial. Ideally we improve documentation on how to build your config / design matrix (@AJQuinn )
• implement tutorial to readthedocs

[matsvanes]

THere are some errors in the GLM spectrum API. the GLM config has description:

config : :py:class:`glmtools.config.GLMConfig <glmtools.config.GLMConfig>`
            The  GLM configuration object.

But the config should actually be a sails.stft.GLMPeriodogramConfig.

[matsvanes]

Fixed in #351