Try testing overloaded mbstring functions in Travis

This _should_ cause the Travis build to fail.
Overloading value is apparently a bitmask
Set mbstring encoding
Fixed for mbstring
Move string length and read to util class

I don't really like Util classes, but these two static methods
exist only to hide PHP braindamage.
Fixed remaining strlen and exception test
Added change log entry
Merge pull request #11 from maxmind/greg/mbstring-fix

Fix when mbstring overloading is enabled