DRILL-3778: Add missed part of DRILL-3160 (making JDBC Javadoc available).
This closes #158
Main:
Configured Javadoc generation (title, package groups, version in headers).
Added link to JDBC page in Drill documentation site.
Edited/fixed some JDBC Javadoc comments.
Added explicit SQLConversionOverflowException to throws clauses for Javadoc
effect.
Added some imports for Javadoc references.
Misc.:
Fixed a couple Javadoc syntax errors.
Fixed POM indentation.
