Auto-detection of non-ascii strings

This patch will automatically use UTF-16 encoding when string needs instead of letting it be in UTF-8 in the file and loose data, as plist format seems to wait for ASCII data. So this put ascii encoding a default, and, if it detects a string with non-ascii characters, will force a UTF-16 value to not loose data. So characters like é, å or even ♫ can be encoded without any loss (these characters suffered from some kind of "conversion" and were not displayed correctly by Xcode) and are now visible correctly in Xcode and `plutil`.

The `sample1.bplist` file has been edited with Xcode to put back a correct © sign and use UTF-16 string for this value (automatically converted by Xcode).