repository: improved error message for conflicting repository name

When trying to add a repository which name conflicts, return an explicit
message resuming the error.

Closes #33
misc: import MongoDB scripts from tsuru

Related to globocom/tsuru#234.
Added usage for backup.bash.
all: clean the database on TearDownSuite's
api: don't rely on error message from the json package

This message changed on tip :-)
db: improve tests

The compiler already guarantees the return type of a function, we should
not make that kind of assertion in the runtime, even on tests.
docs: improve backup documentation

Don't provide specific installation instructions for operating systems,
and create some sessions (preparing for the MongoDB backups
documentation).
docs: mention script for backing up mongodb in backup page

Also refer to the database healing script.
misc/mongodb: sync scripts with tsuru
docs: update documentation for mongodb healer
all: added license headers to files
check-fmt: give the proper credits
db: introduce Connect function

Instead of using init to connect to the database, users of this package
must now call Connect. Connect uses information from the config file to
determine to which database it has to connect.

Related to #59.
db: added basic documentation
all: using db.Connect to connect to the database before using it

Closes #59.
bin: setup the database on SetUpSuite

Fix the builds. Sorry about that.
travis: irc notification
requirements: added circus
misc: created circus.ini file
.gitignore: ignoring *.pyc