Convert "safe" write option to "w" for drivers 1.3.0+

This will help avoid deprecation notices in drivers 1.5.0+ (for code still using "safe").
Allow 1.5.x driver versions in composer.json

Closes #144
Merge pull request #145 from doctrine/1.0.x-convertSafeOption

Convert "safe" write option to "w" for drivers 1.3.0+
Merge branch '1.0.x'

Conflicts:
	lib/Doctrine/MongoDB/Collection.php
	lib/Doctrine/MongoDB/GridFS.php
	tests/Doctrine/MongoDB/Tests/CollectionTest.php