Report printf specifiers from Service API

Close #174.

The PR seems to cover the most important cases: 
 - regular vs. verbatim vs. triple-quote strings
 - single-line vs. multiline strings
 - built-in printf functions vs. printf-like user-defined functions

One thing isn't very nice: ASTs don't preserve information whether a string is regular, verbatim or triple-quote. Consequently, we have to lookup the source string to recover that information and create correct offsets for reporting ranges.
Please revert these changes if they aren't needed (We want to minimize all FCS diffs if possible, to minimize divergence from the F# compiler codebase. So only make textual changes if totally necessary)