Updated typings to include default export

[keego]

Closes #89

This modifies the typings so that package 'vue-rx':

1. now exports VueRx by default
2. no longer exports install, which wasn't actually being exported anyway

VueRx should now be able to be imported as mentioned in the npm section of the README:

import Vue from 'vue'
import VueRx from 'vue-rx'

Vue.use(VueRx)

[keego]

@yyx990803 should be good to go. Can I get a review?

[yyx990803]

Thanks!