Improve WriteError utility

* WriteError accepts multiple arguments like Printf
* WriteError automatically adds a trailing `\n` if there isn't one
Manage PR assignment with `pulls take` and `pulls drop`

* `pulls take` assigns a PR to the current user
* `pulls take --steal` steals a PR from its current owner
* If not authorized to take, it posts a "#volunteer" comment

* `pulls drop` unassigns a PR from the current user (if it owns it)
Move maintainers code into new file

Refactor and cleanup some funcs
Rewrite get maintainers logic
'pulls checkout' is safe and re-entrant

* Can be safely called from any current branch
* Can be safely called several times in a row
* Re-pushes are always taken into account
* No risk of merging into an old local master
* Doesn't impose a branch naming scheme: you get an anonymous branch and
decide what to do with it.
Merge pull request #100 from shykes/pulls-checkout-reentrant

'pulls checkout' is safe and re-entrant
Merge pull request #102 from crosbymichael/reviewers-fix

Rewrite the maintainers and reviewers logic
Comment on a PR with `pulls comment`
Merge pull request #103 from shykes/pulls-comment

Comment on a PR with `pulls comment`
Use fmt.Printf() instead of fmt.Fprintf(os.Stdout, ...)
Code style: unify the use of WriteError
Rename WriteError to Fatalf for clarity
Merge pull request #106 from shykes/style
Add more information to pull request view

This adds if the PR is mergeable.  If it was merged it adds who merged
it, what time, and the merge commit.

[solomon@docker.com: rebased and resolved merge conflicts]
Signed-off-by: Solomon Hykes <solomon@docker.com>
Close a PR with `pulls close`

Signed-off-by: Solomon Hykes <solomon@docker.com>
add --assigned & --unassigned

[solomon@docker.com: rebased on master]
Signed-off-by: Solomon Hykes <solomon@docker.com>
Disable coloring if stdout is not a terminal

Signed-off-by: Solomon Hykes <solomon@docker.com>

This allows piping and redirecting the output of `pulls` without mixing
in term control characters garbage.
Add a comment when taking an PR

The comment format is `#assignee=<USER>`

Signed-Off-By: Solomon Hykes <solomon@docker.com>
remove filters from pulls
Merge pull request #110 from shykes/pulls-close