[Fix] loadSVGFromString receives raw buffer instead of string when loading SVG file from disk

According to [the NodeJS documentation](http://nodejs.org/api/fs.html#fs_fs_readfile_filename_options_callback), `fs.readFile()` returns the raw buffer if no encoding is specified. Since `fabric.loadSVGFromString()` expects a String as the first parameter,  the call would break the canvas loading if a SVG file is involved. This PR fixes that, transforming the buffer into a String (assuming `utf-8` encoding, which is [the default](http://nodejs.org/api/buffer.html#buffer_buf_tostring_encoding_start_end)).

**Note**: If multiple encodings support is something desirable, it may be wise to add an optional parameter somehow, which would specify an encoding other than `utf-8`.