Accept all types as boolean

[brad]

Based on this discussion on the mailing list:
http://rocklinux.net/pipermail/openscad/2011-September/001535.html

This commit gives a boolean value to other types and allows their use in "if" statuments.
I used the same logic that PHP uses to determine if a type is true or false:

the boolean FALSE itself
the integer 0 (zero)
the float 0.0 (zero)
the empty string, and the string "0"
an array with zero elements

[whoisjake]

just throwing my own opinion in here... but I like a very simple and explicit definition of boolean values... at the very least, one of the boolean values.

In Ruby... the rules are simple: symbol/keyword false is false, nil is false, and anything else is true.

[brad]

whoops! those commits don't belong in this pull request. I'll fix it.

[brad]

Fixed, now these last two comments from me will look completely nonsensical.

[kintel]

This was fixed in 6659ad4