add resizable option, Extract resizing code into separate directive

this pull request resolved https://github.com/alalonde/angular-scrollable-table/issues/18, https://github.com/alalonde/angular-scrollable-table/issues/19, https://github.com/alalonde/angular-scrollable-table/issues/20. 

1. User can append 'resizable' to table header to enable resize feature for this column.
2. Moving all resize functions into resizable Directive. So far, the resizable directive is depend on sortableHeader directive. Maybe we can create a 'abstract header directive' to let sortableHeader and resizable directive to depend on.
3. using browser to calculate column's width when the Table resizes until User resizes any column.