Add support for pricing modules for tracking pricing

[ngardiner]

Adds support for pricing modules which would provide pricing data to TWCManager for use in policy decisions.

• Add Web API function for fetching the current pricing data
• Add Web API visibility of current import / export
  • Add both current import and export price to the default dashboard
• Add background polling of pricing data from one or more defined pricing modules
• Add static pricing module, which allows configuring of pricing for environments without dynamic pricing queries
  • Significant additional tunability required
• Modify policy to use pricing data
  • Add a maximum price value to the policy section of the configuration
  • Stop charging if price reaches defined maximum price
  • Stop charging if export price exceeds import price

Note that a small amount of cleanup is included in the PR, which isn't directly related to the power pricing support:

• Reordered several functions in TWCMaster which weren't following convention of alphabetical order
• Removed the status column from README.md for Status modules, which have all been supported since 1.0.1

[ngardiner]

This PR is impacted by #331 - Just a heads up for now.