Update the CONTRIBUTING and README documents for the github move

Adds .md extension on the readme so github knows to treat it as markdown.
Fix location of pre-push script mentioned in CONTRIBUTING
Trivial import ordering fix to make go fmt happy
Refresh session before checking replica set status
Merge pull request #1 from bz2/update_contributing

Adds .md extension on the readme so github knows to treat it as markdown.
Merge pull request #3 from voidspace/initiate-refresh
Made clear how to make the precommit hook

There is a pre-commit hook set up step in the docs that references
a relative path. A line exemplifying where that command is run from
has been added to address the frequent question of "what is this
relative to"
Update reference to bzr to use git instead
Merge pull request #7 from johnweldon/docs

Update bzr instructions with git instructions in README.md
Migrated from LP
Merge pull request #9 from dimitern/500-add-networks-constraints

state/constraints: Networks constraints added

This adds "networks=..." constraint in the constraints
and state packages. The ability to specify networks
to include/exclude when selecting a machine to deploy
a service's unit with specified constraints. Both
positive and negative entries are supported and the
value of "networks=" is a comma-delimited list of
(juju) network names. Negative entries have "^" prefix.

Examples: networks=logging,^db,storage,^dmz

Means: use machines (if possible) on the "logging" and
"storage" networks and not on "db" or "dmz" networks.

No changes to cmd/juju/deploy or other commands yet,
this will come in a follow-up.

NOTE: This branch was migrated from Launchpad and
was already reviewed at https://codereview.appspot.com/93670046/
Merge pull request #4 from perrito666/contributor_doc_updates

There is a pre-commit hook set up step in the docs that references
a relative path. The usual confusion is because the relative path
in ln is relative to where the link is created not to the current
path and therefore people thinks they have a folder missing.
A line exemplifying where that command is run from
has been added to address the frequent question of "what is this
relative to"