add an API to set the default projector of embedding

[youkaichao]

Now, when I'm using embedding, the projector uses PCA by default, and I have to wait for the PCA to finish computing, then switch to t-SNE. It'll be so nice if I can set the default projector to t-SNE because PCA on large data sometimes is time-consuming and time-wasting if I only want to use t-SNE!