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
Is the risk of collisions sufficiently low? Both with other links and predefined host MACs.

Might we want to adapt another OUI for random MACs to reduce risk of collision with predefined MACs?

@lantz what do you think?