Added support for FileWriter.truncate()
Added support for FileWriter.seek()
Set FileWriter.seek() according to specification
Removed invalid span tag
Added support for seek() and truncate() and improved write()

Based on code from FileWriter polyfill at ShareIt! project
Adjusting code to Google's JS style guide
Merge pull request #6 from ShareIt-project/master

Added support for seek() and truncate()