PHP-1237: Add extra check for underflow, which theoretically could happen.

Can't find a test case for this though.
Merged pull request #766
Fixed test runner not skipping on 2.6/standalone.
Merged pull request #768
Use %d for ping value, as on Travis it's not always 0.
Merged pull request #767
PHP-1272: Fix segfault with bson_encode() and MongoDate
PHP-1273: Fix bson_encode() handling of MongoTimestamp objects

Previously, MongoTimestamp objects were serialized as BSON dates, which also lead to property access errors.

Additionally, this changes the code to use a static buffer, which is consistent with other fixed-size types (e.g. dates, integers).
PHP-1277: bson_encode() should return empty string for min/max keys

MongoMinKey and MongoMaxKey do not have values and are only differentiated by the type byte in a document (like null values).
PHP-1272: Implement missing tests for bson_encode()