Submitted PR for an actionloop based runtime for Python

[Michele Sciabarra <mi...@sciabarra.com>]

Hello all,

I just submitted PR #44 for an actionloop based Python Runtime. In my tests, it is only slightly slower than the top ones (1000 actions in 0.21 against 0.17 for the go and PHP based runtimes).
I tried to make it as backward compatible as possible.  It passes all the existing tests although I had to modify a few of them to adapt to some (IHMO minor) differences in the runtimes.

Notable differences are:
- at init time it does not log the errors, but returns compilation errors in the answer, in format {"error": "compilation error"}
- if the action produces at runtime an `{error` it still returns error code 200, not 502

In the first case, it happens because actionloop does not log anything at init time. I could change it if needed but this way you have a consistent behavior in actionloop based runtimes.

In the second case, it is a bit tricky. I found a test expecting that when you raise an exception in the action, I should return an "{"error:"...}" and set the error code to 502. Unfortunately, because the only communication channel among the action running a loop and the web server is a pipe, I should parse the answer (that can be large)  just to find it has a key "error" for each request and I do not want to slow down the runtime just for setting this error code.

Please revise the PR#44 for the Python runtime. Note also that before all I need someone merges the PR#4193 on incubator-openwhisk because test data are placed in the main repository so far and I need a python37_virtualenv.zip there.