astakosclient: Fallback to Python's json package

If simplejson is not installed, fallback to python's default 'json'
package. This is needed because 'simplejson' is not available in some
distributions (i.e. CentOs 7).
pithos: Change quota error handling

Pithos backend used to catch errors generated by the AstakosClient
and in those cases would return 413 (Request Entity too Large).
However, quota limit failures are not the only failures can occur.
Therefore, 413 responses are not always appropriate.
To fix this problem, let the respective methods fail with an
AstakosClientException which is handled by the common api decorator.
Small typo fix

closes grnet/synnefo#198
docs: Update CentOS 6.5 installation guide for 0.16

closes synnefo/grnet#192
cyclades: Fix mishandling of MAX_CIDR_BLOCK setting

Fix mishandling of MAX_CIDR_BLOCK setting. The CIDR range should be
[MAX_CIDR_BLOCK, 29] instead of (MAX_CIDR_BLOCK, 29] that was until now.
pithos: Several PEP8 and test fixes
pithos: Apply metadata/group limits for the resources

This commit sets a maximum metadata number per Pithos
resource as well as a maximum group number per account
and a maximum allowed number of group members.

More specifically:
* Introduces two new settings for Pithos:
* the maximum number of metadata each resource
  (account/container/object) can have,
  the maximum number of groups each account can have
  and the maximum number of members each group can have.
* Adds checks to enforce the above limits.
* Introduces the LimitExceeded exception raised by the backend
* when these limits are infringed.
* Refactors the backend implementation of the group/metadata
  update in order to reduce interaction with the database.

The affected backend methods are:
* update_account_meta
* update_container_meta
* update_object_meta
* register_object_map
* copy_object
* move_object
* update_object_hashmap
* update_account_groups

Applications, other than Pithos+, that use the Pithos backend
should put a special care on handling these situations.
plankton: Handle limit exceptions.

Pithos backend enforces limits on the number of the resource metadata.
If these limits are exceeded a LimitExceeded exception is raised.
Plankton should catch these exceptions and fail with BADREQUEST (400).
docs: Update storage API
pithos: Fix group issues

* Fix encoding issues
* Provide the appropriate tests