Development version

This are the changes in the current development version of BootCaT:

• FEATURE: a log file (containing errors and warnings) is now written to the corpus directory at the end of the corpus creation process;
• FEATURE: downloaded files are now assigned an extension based on the detected mimetype, (previously they were assigned the same extension as the URL they were downloaded from);
• BUGFIX: Fixed a bug that made the Windows version of BootCaT crash while downloading files with colons in the filename extension;