adding cluster user for query execution

Fixes:
	1. User related configs will be resolved when session is opened. Cluster user, queue etc. belong to this category
	2. from cli, current logged in user will be passed as cluster user
	3. Domain can be specified on grill level and will be passed to authentication providers as needed
	4. Config will be resolved by config loaders. This can be chosen from the ones provided, or can be custom. Providing three such implementations. Each has its own config.
	5. Test cases for each loader. And test cases for verifying cluster user and loggedin user are set in session.
reviewed comments and made changes. Nothing major.
Merge pull request #125 from prongs/cluster_user_clean

adding cluster user for query execution
Fixing small issues
	1. Correcting default values for database config loader
	2. Removed Exception Constructor because java 6 doesn't have that constructor for the super class (RuntimeException)
	3. Regenerated apt file
Merge pull request #126 from prongs/cluster_user_clean

Fixing small issues
Add service provider api to get instances of various services
Merge pull request #128 from jdhok/develop2

Add service provider api to get instances of various services
Merged with develop