Error handling improvements

* Add http_code attribute to MistError subclasses
  The mapping of MistError's to HTTP responses is now done
  in the exception's definition.
* Improve and clean up exception handler.
* Make exception handler gracefully handle generic exceptions too.
Fix missing 's' in char substitution
Merge pull request #61 from mistio/fix_typos

Fix missing 's' in char substitution
Merge pull request #60 from mistio/exc

Error handling improvements
Add file locking mechanism to make thread safe.

Implement a FileLock mechanism that uses soft links to create a lock.
Create new OODictYamlLock class that uses FileLock and make User
class a subclass of OODictYamlLock.

Every call to lock_n_load locks db.yaml by creating a db.yaml.lock
soft link (atomic operation). If link (lock) already exists, call will
block up to a defined timeout after which it will break the lock.

Also, when saving, data is dumped to db.yaml.tmp file and then moved in
position (atomic operation) to avoid a thread reading halfwritten data.
Merge pull request #62 from mistio/lockfile

Add file locking mechanism to make thread safe.
Add support for custom region in Openstack
Add openstack region support for frontend
Merge pull request #63 from mistio/openstack_region

Add Support for custom Openstack Region
Merge branch 'master' into staging
Automated build of mist.js
Merge branch 'master' into graphsZoomInOut