[MRG] DOC fix formatting of Attributes sections in documentation

The Attributes sections of class documentation are currently formatted poorly and [appear after](http://scikit-learn.org/dev/modules/generated/sklearn.ensemble.ExtraTreesClassifier.html) See Also, References, etc.

This PR uses the same formatting code for Attributes as for Parameters, and places them together, as they are conventionally in docstrings. The inconsistent convention to surround attribute names with backticks was [critiqued](http://sourceforge.net/p/scikit-learn/mailman/message/30617924/) by @vene in March 2013; this PR gets rid of that inconsistency. It also fixes up a few misformatted Attribute descriptions (missing space before `:`) and Returns descriptions (surrounded by backticks).