db::select now returns data using out-reference
rather than function result, to avoid useless map copies.
... but this doesn't affect neatly performances...
Lazy-loading of instances inside collections.
... may improve perfs of joins...or no.
Removed transfer objects when fetching SQL selects
=> Implemented an "abstract" interface in dbi.h for selection and access to results,
   that allows direct copy of fetched data into table<T> results.
Removed  duplicate test code from table_unittest
column.h with no dynamic allocation (T* -> T)
null semantic is know controled using flags
removed public data members, thus allowing consistency of flags
when accessing data.
Try: fetch selects using an sqlite_exec callback,
that directly transfers data into the returned collection of results.
=> fetch times are even worst (3200ms -> 6100ms) :o(
Result collections are now dynamically allocated,
thus avoiding copy of complex objects in "return" statements.
Fix:forgot changing return of select() to pointer,
in the table<T>::select() version that do not use DB callbacks.
Implemented abstract "select" interface for mysql
Partially fixing issue #1, but still no default
values for TEXT type => Kept nasty "VARCHAR(2048)"
Mysql support for TEXT with default value,
(see issue #1)
Added some "should_fail" tests in table_unittest.
Impl:Quoting of table and field names in requests,
using polymorphic method in dbi and its concrete implementations.
=> Fixes issue #1 (mySql support)
Lorm::connect now throws for bad connection string
and this is asserted by a new test in table_unittest
Restored "lazy-loaded" collections,
(=> exploring optimization tracks "à la " ODB...)
Fixed: reference::exist() now returns a reliable info.
Fixed columnt<T>::operator <
Used sqlite3_data_column instead
Restored non-lazy loading for "T::find()" collections
(still remains lazy on join loadings)
Impl. caching of prepared statements,
to avoid re-compilation of statements
(only those with one "?" param).
gain : 40% more speed on my MBP
Removed debug traces from sqlite_statement
Encapsulated "T::identity_col_" within a getter,
to force "T::register_class()" call when referring to T identity.
(and before any T instance has been created).
Close issue #17: Support for column<bool>
Fixed: declaration of column<bool>as_sql_litteral()
=>  was missing
Many refactoring, see below...
* Accessors to "to-one" are now implemented in the owning class,
  and removed from reference<T>, where they initially were implemented as "operator ()"
* Added macro for "to-one" member declaration
* Added macro for "to-one" accessors implementation (get/set).
=> all this stuff aims to prepare "join expressions" impl. (see issue #19)