PATCH: Add support to select branch for sSVN

Created an attachment (id=5093)
Patch to add support for selecting the branch using the ROS_BRANCH variable

This patch adds selection of the branch for sSVN using the ROS_BRANCH variable.

For example, setting ROS_BRANCH to "arwinss" (without quotes) will allow sSVN to operate on the ARWINSS branch.

If ROS_ARCH is set to "amd64" this will override ROS_BRANCH

NOTE: ROS_BRANCH is "recyclable" - by that I mean you can use it for multiple sSVN executions without it being changed, since this new sSVN modifies its own internal _ROS_BRANCH variable.

A nice side-effect of this patch is that it also refactors redundant 'if "%ROS_ARCH%" == "amd64" () else ()' style blocks, making the code a little smaller and neater.

Patch by Adam Kachwalla