Added  documentation for `fit_info`
Added test for param_cov
fix error in errorfunc usage revealed by tests
change param_jac to cov_x, because it's not really a jacobian
Changing FITS table write function to have default value of Nan for float type cols
Changing default value for float columns from 1e20 to Nan in fits table write function
Changing the float comparison for filled value to np.allclose to enable Nan masking for float32 as well. Also added test cases for float64, and float32
Update docguide.rst

typo fix
Restructure front documentation page to emphasize contributions rather than development
Example of reworked documentation aimed at beginners
Clarify language in several places
Change line breaks to keep length under 80 characters
Fix typographical errors
Add caveats about cases when setup.py develop will fail
Clarify warning about need to reactive development astropy

New wording suggested by @mdboom
Fix line that was too long
Add summary of virtual environment commands for anaconda and virtualenvwrapper
Condense git installation/configuration instructions
Fix typographic errors
Minor wording changes and link updates