fixes #210
Print class uses String so provide a forward reference in cases where the header hasn't been pulled in.

(the string header also depends upon the print header, so we can't just pull in spring_wiring_string.h since that will have no effect.)
fix for segment fault when converting values to binary. (The buffer size was set assuming a 16-bit integer, not 32-bit.)
fix for converting `INT_MIN` to string, cannot negate in that case because `-INT_MIN` isn't representable in 32-bits.
unit tests
gah..travis build couldn't find INT_MIN. strange it compiled locally. Would be great if travis allowed you to host your own docker vm, then we'd have a portable build environment, avoiding these build discrepancies.
proper fix for INT_MIN - it was in spark_wiring_string.cpp, not in the test case
need a local build env that is a mirror of travis.
Merge pull request #292 from spark/feature/string-base-fix

type-safe radix constants