decrease specificity of selectors and .less cleanup

removes .gridster prefixes so a user applying their own class
to a gridster-item element no longer needs to prefix their own declaration with
.gridster for their changes to  override the library's css
(text-align for example)

also combines rules that were previously separated and uses less nesting for
a cleaner file
Merge pull request #108 from kdoh/kdoh_less_specificity

decrease specificity of selectors and .less cleanup