New units and user-customisable units

Loads of new features!

• Bugfixes, including a big one in the parser (leading to incorrect conversions)
• Lots of new currencies (155 total currencies now supported), fixing #1 and #3
• A couple of new built-in units, partially fixing #4
• User-defined units — You can now add your own unit definitions (use keyword convinfo)
• Configurable number of decimal places in conversion results
• Automatically update from GitHub
• New icon