refactor(progress): Rename progress attribute from gantt-task-progress

progress is a directive in ui.bootstrap, and conflicts when this module is imported with angular-gantt.

This renames progress attribute to gantt-task-progress-value to avoid this conflict.

Close #205
fix(filter): Fix invalid filtering when filters are undefined

When fiters are undefined, rows and tasks were added multiple times, causing various issues.

A clone is now made from rows to filteredRows and tasks to filteredTasks, solving this issue.

Close #206