* config: not properly loaded error
When a wrong config file was used, it result in a panic.
Adding some check condition to validate the Unmarshaled configuration
before using it
fixes#134
* check if datasource is set
* move error locally instead of utils/errors
Clair will now use a YAML configuration file instead of command line
arguments as the number of parameters grows.
Also, Clair now exposes a Boot() func that allows everyone to easily
create their own project and load dynamically their own fetchers/updaters.
