Add [NumberIsFloat] attribute for PlayScript

This class level attribute instructs the compiler to use single
precision floats instead of doubles for the PlayScript Number type. It
is applied recursively to all Number types in the class.
Add PlayScript implicit conversion from double to float
Add null checks to each visitor Accept function
Add PlayScript implicit conversion from float to int/uint

Previously all Number types in PlayScript were doubles; but with the
addition of the [NumberIsFloat] attribute, Numbers can be floats, and
the conversions are required.
Fix typo in NumberIsFloatAttribute.cs
Fix typo in class.cs
Merge pull request #269 from playscript/cbroglie-floats

Add support for single precision Numbers in PlayScript