Add GSS support.

Second patch concerning the support of GSS in GWT. This patch
introduce the generator that will parse and compile a GSS file
to a css string and generate the implementation of the
corresponding CssResource interface. GSS support is disabled
by default. To enable it, you have to set the configuration
property "CssResource.enableGss" to true.

Change-Id: Iae743842c5707c0231b1c4eb290f7914b6f7a501
Add GSS support in UiBinder.

Third patch concerning the support of GSS in GWT. This patch
introduces a new attribute named useGss that you can use
in order to enable GSS in the inline style of UiBinder

Change-Id: I7e6f907c3ffa4a4c8f642786506de2f9c4107e85
Fix maven tag inserted in bad position.

fixes change Idc43b626e67de86559548b4fdae97481907e9ca3

Change-Id: Ifdbfe3477a6ef9c9ff9fde3c339c6ca6c4a3189e