add base2string and string2base to string.k

@andreistefanescu please review.

Fixes #573
```
dwightguth@rvwork-1:~/k$ grep -i string2base -R .
./lib/maude/lib/builtins.maude:  op string2Base : #String #NzNat -> #Int .
./lib/maude/lib/builtins.maude:ceq string2Base(S:#String, N:#NzNat) = I:#Int if I:#Int := String2Rat(S:#String, N:#NzNat) .				  
./lib/maude/MaudeHooksMap.properties:\#CONVERSION\:string2Base=\#CONVERSION\:string2Base
./include/java/hooks.properties:\#CONVERSION\:string2Base : org.kframework.backend.java.builtins.BuiltinStringOperations.string2base
./.git/logs/HEAD:7e26ab2763545262b71c920119a606bbee48de13 ed6e8ddc680019750ccd75c3c7711a1a01cd7c4e Dwight Guth <dwightguth@runtimeverification.com> 1403193984 -0500	commit: add base2string and string2base to string.k
./.git/logs/refs/heads/base2string:7e26ab2763545262b71c920119a606bbee48de13 ed6e8ddc680019750ccd75c3c7711a1a01cd7c4e Dwight Guth <dwightguth@runtimeverification.com> 1403193984 -0500	commit: add base2string and string2base to string.k
./.git/COMMIT_EDITMSG:add base2string and string2base to string.k
./src/javasources/KTool/src/org/kframework/backend/java/builtins/BuiltinStringOperations.java:    public static IntToken string2base(StringToken term, IntToken base, TermContext context) {
dwightguth@rvwork-1:~/k$ 
```