Fix dmlc::strtof()

[hcho3]

Previous implementation of locale-independent parsing (#455) was faulty:

1. It broke builds for MinGW-w64 + Windows 7 target (Fixes recently introduced error with _create_locale and msvcrt #478)
2. It doesn't parse 0 correctly (Locale-agnostic parameter parsing #455 (comment)).
3. The use of locale functions adds too much complexity, which goes against the spirit of dmlc-core as basic building blocks.

This PR rectifies the situation by re-using existing implementation of strtof(). In particular,

1. dmlc::data::strtof() is moved to include/ directory.
2. Add template parameters to the strtof() implementation to add range checking and to implement dmlc::strtod().
3. Add a better test for float parameter parsing.

Also, the locale functions have been removed.

Closes #478.

[hcho3]

@trivialfis I've removed platform-dependent code for the sake of future maintenance burden.

[hcho3]

TODO. Add docstrings for all public functions in include/dmlc/strtonum.h.

[KOLANICH]

Works fine, thank you.

[hcho3]

@KOLANICH Glad it solved your problem.

TODO: use libc-test to rigorously test dmlc::strtof() and dmlc::strtod().

[hcho3]

It turns out dmlc::strtof() won't be C standard compliant, since it cannot parse INF/NAN and cannot correctly handle long decimals. Will need to clearly document limitations.