Various fixes and improvements

[rekmarks]

Fixes #5, Fixes #68, Fixes #99, Fixes #138

Changes in Detail

• Add wallet_getPlugins RPC method
  • This returns metadata about permitted plugins to the caller
  • Indicates whether plugins are installed or not
• PermissionsController
  • Do not reinstall plugins if they're already installed
  • At this point in beta, plugins should just be removed using the buttons we expose before reinstalling them
  • New methods: getPermittedPlugins, clearDomainMetadata
  • Improve section demarcation/commenting
• PluginsController
  • Add teardown for plugin onMetaMaskEvent listeners
  • We weren't removing these event listeners previously!
  • Added new methods: _removeAllMetaMaskEventListeners, _removeAllMetaMaskEventListeners
  • Extensively modified: _createMetaMaskEventListener, _generateMetaMaskListenerMethodsMap, _eventEmitterToListenerMap
  • Add doc comments for all but the most trivial internal methods
  • This is where much/most of the diff comes from
  • Stop storing plugin.approvedPermissions
  • Always get current plugin permissions from PermissionsController
  • Does not affect initialPermissions
  • New method: getSerializable for getting plugins without getting e.g. their source code
• Tear down plugin providers
  • See method closeAllConnections in metamask-controller.js and its use in PluginsController
• Miscellaneous
  • Return userRejectedRequest error from wallet_enable if no permissions were granted
  • Rename siteMetadata to domainMetadata everywhere
  • To reflect that it can refer to not just sites, but also plugins, and who knows what else in the future