fix collection_for in Implement lazy associations +  Implement callbacks for Models (#16)

contains 
1)
https://github.com/unplugandplay/guacamole/commit/1dd3ff6465712ffaec6e3e9e4cdf49c62e439667
https://github.com/unplugandplay/guacamole/commit/b868de5038b31af12f314c5f1472b6ee50221960
https://github.com/unplugandplay/guacamole/commit/61345bdc547f4166d40c14b8f93e1d512da3321f

for  lazy associations

2)
https://github.com/unplugandplay/guacamole/commit/1c636f47935e084740b69f7b28e956494e45cd62
https://github.com/unplugandplay/guacamole/commit/96a85ae317f536e5523082ee23011a50da116a8c
https://github.com/unplugandplay/guacamole/commit/e42df2a1c7dccb99f4f5f1b7239692142f3e1fbe
for callbacks