WriteFloat implementation for type double on ARM Targets

[TheTh0r]

Double currently is casted to float and the writeFloat function is used in the IOStream.

(See TODO in /src/xpcc/io/iostream_float.cpp line 93)

[salkinium]

And floats cannot be formatted by number of decimal points or similar, because it's all hard-coded.

Also interesting and mildly related to the topic is an alternative implementation of the OStream:
http://probablydance.com/2014/08/30/format_it-iterator-based-string-formatting/