extension/bugfix for ym-linear-gradient

Sorry in advance, if this isn't the right way to suggest fixes/discuss problems. First time I try this in github :)

When trying to use the ym-linear-gradient-mixin in my project, I occured several problems:

background: linear-gradient($direction, $start-color,$end-color);
 was compiled to 
background: linear, to bottom, #FF00FF, #FFFF00;

I'm not sure what exactly caused the compilation problem, but using the "unquote"-function together with Interpolation ( #{$...} ) fixed the problem.

Furthermore I added a fallback for older browsers (i.e. opera mini mobile browser) to avoid some undesired situations I occured (white color above white background if no gradient is applied). 

Last but not least, I changed "background" to "background-image" to avoid complications between color-fallback and gradient.