In Shell.eval, add quotes around float values

Since bitlash does not understand floats, it gets confused with the dot.
By adding quotes around the rendered float, it can at least be processed
(e.g. in a custom report).

This changes ValueToQuotedString::append to use the String constructor
explicitely, since there is no implicit conversion from floats to
String.
Merge pull request #215 from Pinoccio/evalfloat

In Shell.eval, add quotes around float values