add exclude modules argument to map

[Vaishaal]

Exclude modules in map based on prefix

Fix to #136

[ooq]

Let's maybe have an example to document this?

[Vaishaal]

Here is an example

library.py:

import big_module # lets say this includes some stupid dependency tree that is like > 100 mb in size
def foo(x):
      return big_module.foo(x)

def bar(x):
       return x

driver.py:

import library 
pwex = pywren.default_executor()
pwex.map(library.bar, [0,1,2,3]) # will fail
pwex.map(library.bar, [0,1,2,3], exclude_modules=["big_module"]) # will work

[Vaishaal]

I've also added a test. @ericmjonas could you see if this is sufficient?

[ooq]

If exclusion does not work, there might still be some exception because extmodule does not really exist right?

[Vaishaal]

What do you mean? In this case extmodule exists right?

[ooq]

Did you import it?

[Vaishaal]

at the top. line 6.

[ooq]

Ah okay, I missed it.

[shivaram]

I think this looks fine to me. @ooq do you want to take one final look ?

[ooq]

Looks good. Will merge.