add is_ajax() method

by checking HTTP_X_REQUEST_WITH in server variable. not a good pactice
but it is working.
fix db return error while updating with no rows affected
new api design
can do one() which will retrieve only one column
works for simple SELECT and JOIN without ON clause
can do ON now
no need this one
not in DataType anymore
encoded where clause
on using encoded statement
add limit clause
return DataModel if traversal class only one
if no traversal class return array add query() to dbconnection

query() is alias of execute_query()
fix insertion
add bindparams to dbconnection->execute*
fix update command
better strategy for constructing where clause
fix one()
don't use prefix in only one table query
log: add var_dump()