FIX error handling in SVM

Didn't work when y not an array.

Added extra check to feature importances
Implemented SelectFromModel meta-transformer
Fixed issues in SelectFromModel
Fixed minor issues and pep8 errors
from_model and __init__ updated
Moved __init__ to top. Test failure fixed?
Fixed type
Removed duplicated codes
Fixed typo
ENH restructure SelectFromModel and _LearntSelectorMixin
Removed check for deprecated .
Added partial_fit and warm_start
Fixed documentation.Removed obfuscating code