Changed the implementation of main.round and main.roundRecursive, also the factor argument changed

The according tests and use cases where also adopted.

The new implementation is a more robust version to round to decimal places that can be found at:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/round#Example%3a_Decimal_rounding

Also instead of a factor, the exported functions now take the number of desired decimal places directly
Closes #9 Add optional precision argument to Glyphset.writeGlyph, the writeGlyph Module and the Constructor of GLIFPointPen

Also, it adds rounding to guidelines, anchors and the transformation
matrices of components and images

see also related #8
bettter test for NaN, added a test for null and a comment
Fix some comment typos.
Allow file not found to raise an exception

Fixes #7