pip requirements files

This pull request adds requirements files for the various Calico components that can be interpreted by `pip`. It also extends `setup.py` to set those requirements as install-time requirements. Finally, it adds an environment variable, `$CALICODEPS`, that allows users to avoid installing *all* the requirements for every Calico component if they're only using one.

Currently I'm missing the dependencies for the OpenStack plugin because I can't work out how to install neutron via `pip`, so I don't know what its package name is yet. I also need to consult with @neiljerram, @plwhite and @matthewdupre to confirm that these requirements are in fact correct. We shouldn't merge until I've done that digging.

Resolves #63.
This should probably be python-iptables>=0.5.0.  When using 0.4.0, I get this trace in felix, but with 0.5.0, it's fine.

ec 24 21:52:14.842223 127.0.0.1 calico-felix: Felix exiting after uncaught exception
Traceback (most recent call last):
  File "/usr/lib64/python2.7/site-packages/calico/felix/felix.py", line 725, in main
    agent.run()
  File "/usr/lib64/python2.7/site-packages/calico/felix/felix.py", line 620, in run
    self.handlers[message.type](message)
  File "/usr/lib64/python2.7/site-packages/calico/felix/felix.py", line 296, in handle_endpointcreated
    self._update_endpoint(endpoint, message.fields)
  File "/usr/lib64/python2.7/site-packages/calico/felix/felix.py", line 583, in _update_endpoint
    if endpoint.program_endpoint():
  File "/usr/lib64/python2.7/site-packages/calico/felix/endpoint.py", line 189, in program_endpoint
    ipv4_intended, self.mac)
  File "/usr/lib64/python2.7/site-packages/calico/felix/frules.py", line 245, in set_ep_specific_rules
    rule.create_conntrack_match(["INVALID"])
  File "/usr/lib64/python2.7/site-packages/calico/felix/fiptables.py", line 129, in create_conntrack_match
    match.ctstate = state
  File "/usr/lib64/python2.7/site-packages/iptc/ip4tc.py", line 369, in __setattr__
    self.parse(name.replace("_", "-"), value)
  File "/usr/lib64/python2.7/site-packages/iptc/ip4tc.py", line 267, in parse
    value = value.rstrip().lstrip()
AttributeError: 'list' object has no attribute 'rstrip'