Removed @baseValue dependency

Removed @baseValue variable in favor of the ability to set and change
the base value for rem conversion on the fly. Base value defaults to
16px. Base value can be set by using a mixin.

html {
    .font-base-size(14px);
}

will output

html {
    font-size:14px;
}

but will also set the base value for future rem conversions to 14px.