First nuget release

This release features several (slightly) backwards incompatible changes (compared to the previous Nuget release) to existing controls:

• RemoveButton has been renamed to CloseButton
• DeleteButton has been renamed to TrashButton

Several controls have changed their namespace:

• FlatProgressBar and CircularProgressBar have been moved to the Metrolib.Controls namespace

A lot of controls have been added:

• ExpandAllButton
• CollapseAllButton
• RemoveButton (its style matches AddButton)
• DownloadButton
• UploadButton

Furthermore, this release features an improved (but still lacking) documentation of most controls.