Add default constructor and open() method

[kirillin]

Sometimes a separate open method is vital.

[coveralls]

Coverage decreased (-0.7%) to 95.882% when pulling 6e86b71 on kirillin:master into 629497e on SRombauts:master.

[SRombauts]

Hello, thanks for this!

I am quite reluctant to have this change; it modifies a lot the behavior/semantic od the Database object... It needs more thinking, as well as more testing

Have you noticed CI complains about lesser code coverage with your change?
It would be because you don't test all the new code paths.

Also, your default constructor should really Initialize all variables.

[SRombauts]

About the test coverage decrease, you would need to test all cases in the new Database::open() method
(see edit: https://coveralls.io/builds/27254814/source?filename=src/Database.cpp)

But actually, it would be a lot better to have my old constructor call this new open method itself, so you would not need any more unit tests, it would be covered by specific existing ones

[SRombauts]

See also #103 dynamically create 'DB' item and use it until close

[tzpeng]

//Add close() method
//Fix sqlite3_close sometimes return SQLITE_BUSY
int ret = sqlite3_close(db);
while (ret == SQLITE_BUSY) {
ret = SQLITE_OK;

sqlite3_stmt *stmt = sqlite3_next_stmt(db, NULL);
if (stmt != NULL) {
	ret = sqlite3_finalize(stmt);
	if (ret == SQLITE_OK) {
		ret = sqlite3_close(db);
	}
}

}