My rationale for this:
1) in case someone is installing I2P on top of an already existing installation
   this will cause I2P to be shutdown
2) no hangs during the installation if i2p is already installed as a service
3) in case the NT service settings are changed in wrapper.config by us, this
   will force the service settings to be updated.
4) versions of I2P < 0.8.7 did not remove the service when uninstalling. If someone
   removed I2P in the past and reinstalls I2P--opting to run as a service--the
   installation process will hang.