buffer: remove float write range checks

Removed range checks when writing float values, and removed a few
includes and defines. Also updated api docs to reflect that invalid 32
bit float values will be allowed to overflow to `+/-Infinity` or `0`.

@piscisaureus I think this addresses everything we discussed.

As a side, currently on my machine (since 22b84e6) when compiling with clang 3.2 for ia32 the operations `writeFloatLE|BE` and `writeDoubleLE|BE` write signaling NaNs. While on x64 with clang 3.2 and x64/ia32 with gcc 4.7 write quiet NaNs. So under that condition, two tests will fail. Right now I'm considering this a non-issue.