improve startup performance

The bottleneck for startup performance has been the system's polling for output after a command is run. The more commands that are run, the slower mininet gets. This pull request is an attempt to remove as many unnecessary commands as possible. I have seen dramatic startup improvements with this, improving startup of a linear 200 topo with user switches from 60 seconds to 35 seconds, and a linear 200 topo with OVS from 120 seconds to 75 seconds. However, the pty branch slows it back down slightly

improvements made:
- I removed a few repetitive commands in this pull request( 'ifconfig lo up' commands )
- I specify a mac address for each interface when the interface is created. This does two things for us:
       - Allows us to set the OUI to A4:23:05( ONLAB OUI ). this fixes #156
       - Allows us to get rid of a few ifconfig commands during link creation, which were causing it to run so slow.
- added a fix for an issue where PIDs are read in a different line than \r\n in 14.04, which @bocon mentions in #322
macColonHex( random.randint(1, 2**48 - 1) & 0xfeffffffffff ) | 0x020000000000 )