Makefile fixes
Make free functions inline.
Bugfix: Partial parsing of PBF file.

The last buffers of data from a PBF file could sometimes be lost when one
thread thought the work was done when it was not.
Add libxml2 dependency to README
Explicitly use UTF-8 on XML output.

This way non-ascii characters are not escaped making the file a little bit
smaller.
Removed -Wno-long-long

In C++11 the long long type exists, so we don't have to suppress this warning
any more.