Prefix sass variables with 'mfp-' to prevent issues with own variables

The SASS variables have currenlty names like $shadow.
If the SASS file is used as a partial the variables names
can collidate with own variable names. Therefore they
should be prefixed to make them more unique.
Merge pull request #395 from sascha-egerer/sass-variable-prefix

Prefix sass variables with 'mfp-' to prevent issues with own variables