Prevent webhook execution from continuing if no Settings are set

Currently, it allowed execution to continue, but would eventually throw
a NPE, as code later on assumes the Settings are set.
Created SettingsService that ensures user can get required resources

Resolves Issue #32.
Updated version to 2.0.1-SNAPSHOT
Had changed classname to SettingsService. Updated atlassian-plugin.xml
Updated pom version to 2.0.1 to prepare for release