Support GCE LB session affinity

Add support for sessionAffinity to the GCE Load Balancer driver.
sessionAffinity has the effect of causing clients to "stick" to a
particular backend server.

Also, trivial documentation fixes:

  * resolve ambiguous type specification in create_balancer
  * fix typo in development doc

Closes #341

Signed-off-by: Tomaz Muraus <tomaz@apache.org>