Replacing 'char *' declarations by 'const char*'."

This allows us to work better withC++ (std::string) strings. This way
we could pass directly (after calling .c_str()) C++ std:strings to
InfluxDb C API functions. Currently I'm doing something like:

    char *c_str = new char[cpp_std_string.length() + 1];
    strcpy(c_str, cpp_std_string.c_str());
Merge branch 'feature/const-char-in-the-api' of github.com:goyox86/influxdb-c into feature/const-char-in-the-api

* 'feature/const-char-in-the-api' of github.com:goyox86/influxdb-c:
  Replacing 'char *' declarations by 'const char*'."