Build and other fixes

• Remove call to dlopen Python lib since python-config --ldflags already includes -lpython;
• Updated RStudio project to use --preclean when installing and building;
• No longer assume Python interpreter is called python;
• Rollback string encoding in raw vector introduced in 1.0.7;
• Added Python 3.7 to Windows CI testing;
• Calling Py_SetProgramName as per Python C API documentation.

Windows and big object support

Avoid tripping maximum R string size when reading large data objects from Python. The JSON string read from Python is stored into a raw vector, which is then read with jsonlite's push parser through a rawConnection.

Should now compile on Windows using the Rtools Python libraries or the standard Python package. Many thanks to Matthew Fidler (https://github.com/mattfidler), his help was invaluable in helping implement and test this.

Added Appveyor tests for the Windows build.

Fixed Python 3 Support

Fixed compilation on Python 3, fixing #33. Thanks to https://github.com/lfdmotta for reporting this.

Updated Travis tests to cover Python 2 and 3, and also current and devel versions of R.

Startup message will now include contents of Python sys.version information.

Documentation and Makevars cleanup

Cleaned up configure.ac and src/Makevars.in to remove unnecessary steps inherited from RPython code.

Fixed issue with logo image in README file in CRAN page.

Updated versions of package dependencies.

Fix for Rcpp 0.12.7 backwards-incompatible change

Fixes #24. Thanks to @davidski for reporting this.

Improvement to string encoding handling

Taking measures to ensure SnakeCharmR will work properly on systems where the native encoding is not UTF-8:

• Using Python API to check if variable is str/bytes or unicode and handling things correctly. In particular, when we read Python unicode values we flag the resulting R string as UTF-8 since that’s what we Python C API we are using will return;
• Ensuring that Python code being executed is converted to UTF-8 and that the Python interpreter is made aware of that according to PEP 263.

Fix to autoconf

Fixed linking and building errors on some platforms (such as Amazon Linux) by updating the
autoconf file to use the correct C++ compiler preferred by R.

Fix to NULL handling

Fix handling of NULL values in arguments, including a workaround for jeroen/jsonlite#130 which was causing inconsistencies.