Add with statement support to blocks

[cbisnett]

Added support for automatically scoping blocks using Python's with statement. For example what used to be written like this:

if s_block_start("myblock"):
    s_string("foobar")
    ...
s_block_end()

Can now be simplified to this:

with s_block("myblock"):
    s_string("foobar")
    ...

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This change is 

[cbisnett]

Actually now that I think about it, the default argument values aren't being passed when calling through s_block_start(). I'll update the branch.

[jtpereyda]

Very cool! Did you do a manual run of unit_test/blocks.py? I don't think it is included in the automatically run tests; IIRC some of them weren't passing, but if the new test you wrote is passing, that's probably fine.

[jtpereyda]

I would try now but I'm having issues on my current system.

[jtpereyda]

I got my system up and tested it. Good to go. Thank you for the contributions! Feel free to add yourself to the CONTRIBUTORS.txt file.