Get rid of velocity and only let css transitions in the core

[dotnetwise]

Perhaps separating the velocity to another plugin package.

The core meteor-momentum should have little or no dependencies and only ship withcss transitions` plugins.

[zol]

We would accept a PR where the velocity dependency is modified to be weak and plugins that use velocity throw an exception about the missing dependency when they're being registered.

[mitar]

We would accept a PR where the velocity dependency is modified to be weak and plugins that use velocity throw an exception about the missing dependency when they're being registered.

I agree.