Add xenrrd package for Xen RRD related operations.
Add pip requirements file
Have a base XenHost class which supports finding VMs
Add 'with' statement (context) support, which login/logout automatically.
Fix module docstring
Include cf and step information
Implement functionality to import Xen VM metrics data into MongoDB.

Move rrd_updates parsing into RRDUpdates class.
Provide shortcut for finding one vm
Fix hardcoded username and password
Have metrics return host_or_vm info
Rewrite a good portion.

Now save data and auxiliary information for host and vm, respectively.
Fix some bugs.
Write docstrings.
Rename import_metrics.py to importer.py
Feature #269: Periodically store metrics data of each VM/host into MongoDB
Begin writing a RESTful service of metrics data, based on bottle
Fix a duplication error in sample configuration
Support start/end query parameter
Make parsers support both 2.6 and 2.7
Stop using metrics name as key because it could contain dot(.)
Fix doc _id and return count of docs
Use lock to prevent same tasks being run simutaneously