Added timeout implementation per-context level

[desertkun]

Javascript can hang python from many places!

ctx = Context(timeout=0.1)
# JavaScriptTerminated
ctx.eval("while(true);") 

# old API still works and raises JavaScriptTerminated
ctx.eval("while(true);", timeout=2) 

ctx.eval("function freeze() { while(true); }")
# JavaScriptTerminated
ctx.glob.freeze() 

ctx.eval("function Freeze() { while(true); }")
# JavaScriptTerminated
new(ctx.glob.Freeze)

Not anymore.

[tbodt]

I went ahead and fixed a few things so I can merge, since this has been sitting around for months. Plus it includes a fix for #22.

[codecov-io]

Codecov Report

Merging #20 into master will increase coverage by 10.45%. The diff coverage is 94.02%.

@@             Coverage Diff             @@
##           master      #20       +/-   ##
===========================================
+ Coverage   69.93%   80.38%   +10.45%     
===========================================
  Files          16       16               
  Lines        1224     1341      +117     
  Branches      192        0      -192     
===========================================
+ Hits          856     1078      +222     
- Misses        240      263       +23     
+ Partials      128        0      -128

Impacted Files	Coverage Δ
v8py/jsfunction.cpp	96.87% <100%> (+4.28%)	:arrow_up:
v8py/jsobject.cpp	90.75% <100%> (+11.54%)	:arrow_up:
v8py/v8py.cpp	89.47% <100%> (+17.91%)	:arrow_up:
v8py/context.cpp	88.1% <88.88%> (+10.92%)	:arrow_up:
v8py/debugger.h	0% <0%> (ø)	:arrow_up:
v8py/debugger.cpp	10% <0%> (+2%)	:arrow_up:
v8py/greenstack.cpp	40% <0%> (+3.33%)	:arrow_up:
v8py/polyfill.h	83.33% <0%> (+8.33%)	:arrow_up:
... and 6 more

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