Change source files to UTF-8 encoding

For non-english operation systems, people may affect by the file encoding of the source codes. On my Windows 8 with CP936 codepage, the commited files cannot be correctly displayed due to some non-unicode characters used, such as Greek letters and degree symbols. This would cause some compilation error due to errorously missing the character after the non-unicode symbol.  Note that in the diff webpage of Github, due to both encoding are used, the modified version may be not correctly display by Github. If viewing the modified version solely, it'll display correctly.