SERVER-12897, SERVER-12894: Various MSI Fixes

1. Fix icon in Add/Remove Programs
2. Community = Install directory is wrong - it includes the word "Legacy"
SERVER-12903: Fix MSI Upgrade

1. Each version of mongo is installed to a unique directory, ie 2.6.0, and 2.6.1, changed it to be the major version instead 2.6
2. Added a MajorUpgrade element to prevent downgrade, and do the proper thing on upgrade

Minor: The Wix uses the same upgrade code for each MSI, these should be unique per flavor