Address problems with constraints reported in #1060 and #1064
Adds not one but two additional 'notes' to the pip install instructions including a warning not to use sudo unless you know what you're doing, and a link to instructions for using virtualenv.  Also added --no-deps to the instructions by default, which I think will get fewer people into trouble.
Add more useful information to testing header -- encoding info, float info and endianness.
Replace link to Stack Overflow answer with a link to the pip documentation--not much better more immediately relevant.
Merge pull request #1107 from nden/constraints_issues

Address problems with constraints reported in #1060 and #1064

There's still a minor issue with constraints which is that after fitting the constraint attributes on the parameter objects are reset to the default values. But the model.constraints object has the correct values. I am going to merge this so it can be used and will open a different issue for this last problem I am aware of. It doesn't affect fitting but can be misleading.
Fixed Gaussian1dModel stddev
Removed redundant parentheses
Fixed some of the Gaussian1DModel tests
Fixed a couple of tests using Gaussian1DModel
Tidied up and fixed equation for Gaussian2DModel
Adds directions for updating the changelog (and some justification for why it exists and why you should add to it)
This should fix the problem reported in #853. I also changed a few log.warning -> warnings.warn in the convolve module, and moved the small bit of warnings documentation to a more prominent place.  While I was in there I added an example to the astropy.io.fits.diff documentation since it was looking a little bare without the warnings documentation before it.
Fixed expression for derivatives
Merge pull request #853 from iguananaut/issue-853

Can't ignore warnings
Fixes #843: Handle resizing of terminal in progress bar
Fix for non-tty-based consoles
Fix typo that was causing the progress bar to break on Windows
Improve color support in Windows. This gets colors working in IPython. We *could* also support colors in other contexts if we add an optional dependency on pyreadline. But for now it's not worth doing that work--most of our users are using IPython. And if not they should be. But just in case I added a note about this on the known issues page.
This should fix the color-print issues in Python2.6 in both the tests and in actual usage
Okay, this version works on Python versions 2.6 through 3.3 both in the
context of our test suite, and in real console use both using the
vanilla interpreter and IPython, though I'm sure there are still
exceptions.

One problem I ran into is that our `FakeTTY` class that we use in some
of our tests actually behaves sensibly in the face of unicode strings,
whereas the real `sys.stdout` does not, at least not on Python 2.6.

Most of the stuff in the `except UnicodeEncodeError`  block is only needed on
2.6, but there's no reason to explicitly make that check.  Finally,
there's no explicit check against bytes on Python 3, but if someone
passes in a bytes object it will blow up eventually anyways so I'm fine
with that for now.