Fix bug in custom_model_1d that meant that parameters set during __init__ were ignored.
Fix a couple of bugs in parameters, specifically with handling defaults.  Turns out Parameters were returning copies of themselves even when being accessed via the class--this is entirely unecessary.  Parameter *descriptors* (unbound parameters) can just return themselves on __get__.  But on bound parameters the default value wasn't being propagated, leading to bugs in trying to implement models with defaults for parameters.
Takes things a little further with default parameter values for custom 1D models by making use of Parameter.default.  Also added in full support for constraints in custom models, and fixed a bug where a ton of globals were being added to the custom model class's __dict__.
Merge pull request #1720 from astrofrog/fix-custom-init

Fix bug in custom_model_1d