Euler/Quaternion/Vector*: Introduced constructor default values

[mrdoob]

Introduced constructor default values in some classes.

[shelbyspeegle]

Just a heads up in case someone else runs into this - we ran into an issue with unit tests failing after this version (r118).

The issue is that some of our minimum-viable-code-to-run tests ended up passing NaN to the Vector3 constructors at some point in execution (our fault of course).

We found out that NaN, and null survive through the default parameter syntax.

Update - looks like exactly why this change was made - awesome!