Update the "Get IGB" page in the user's guide

The "Get IGB" Page in the User's Guide is now out of date since abandoing Java Web Start.

The information that is currently on the page should be moved to a future page called "Running Older Versions of IGB"

The page should replaced with text and images that walk through the new download process.

There will be a new section for:

*Requirements:
-Linux requires Java 8.1 while Windows and Mac do not
-Mac requires Version 10.7.3
-Windows requires Vista or Later (32-bit and 64-bit)

*Memory:
-Discuss the new memory allocation and the "vm options" for power users

Text can be taken directly from here:
https://www.biostars.org/p/128949/