improves field type suggestion

The previous version of Hyokai can not correctly suggest field type of columns which contain
negative integers or real values represented by exponent notation (e.g. -1, 12e3, etc.).
They were suggested as string type. This change improve the above issue.
replace fromLocal8bit with fromUtf8. we assume that the encoding of input files is UTF-8.
Merge pull request #6 from east301/real-value-string

improves field type suggestion