decrease specificity of selectors and .less cleanup

removes .gridster prefixes so a user applying their own class for custom styling to a gridster-item element no longer needs to prefix their own declaration with .gridster for their styling to take precedence.

for example:
```
<div class="my-item gridster-item">
</div>

/* this is the ideal declaration for my own custom class, but doesn't get applied as the library css is more specific */
.my-item {
  text-align: left;
}

/*
this gets the job done, but isn't very ideal. i've now created a more specific declaration relying on a gridster class in my own css.
*/
.gridster .my-item {
  text-align: left;
}
```

also combines rules that were previously separated, removes empty rules, and uses less nesting for
a cleaner file.