Add RawJS construct for embedding raw JS and define global instances

[almarklein]

Fixes #279, possibly makes #275 easier, helps #104 while honoring #259.

Write raw JS:

RawJS('$')(self.node).datepicker()

...

def foo(x):
    res = []
    RawJS("""
        for (i=0; i<10; i++) {
            ...
        }
    """)

Define globals in app, ui:

# in m1.py
some_module = RawJS("require('some.module')")
foo = RawJS("[]")

# in m2.py
from m1 import foo  # hooray, we can share a constant in JS!