Added DQSharedQuery::isNull() and deprecate DQSharedQuery(DQConnection connection)  constructor

The null object concept has been documented in DQSharedQuery.
DQConnection - Defined the null state and added isNull() function.
DQModel - Do not set default connection in constructor. 

When a function require the access of connection , it will check is current connection null , if yes , then it will set it to default connection.
DQConnection -  Changed the behavior / definition of defaultConnection() function for improve the multiple database access's coding affort.

Now the defaultConnection() will return the default connection for specific connection instead of a global default connection
DQConnection : Deprecate setToDefaultConnection() , and replaced by setDefaultConnection(DQModelMetaInfo*)

Change the default connection for specific model
DQSharedList::isEmpty() is changed to const
Added Ubuntu/Debian packaging script
mv src/dquest.pro => dquest.pro
Updated README.txt for how to build Ubuntu/Debian package
Updated the changelog to add the 0.2 release information
Added src/priv for holding private class , which should not be usable from user.
DQSqliteStatement is moved to private section
Added tests/installtests : Verify the installed dquest
tests/installtests/models.h - User model changed to use DQListWriter to prepare for initialData()
Added new abstract class : DQEngine - Database engine (Abstract class)
Merge commit 'bd460933d86fd19e0fdefeac59e00b2ff9b8d9b6' into v0.3

Conflicts:
	debian/rules
	src/dqconnection.cpp
	tests/sqlitetests/sqlitetests.pro
	tests/unittests/sqlitetests.cpp