Clean up #include ordering and add missing ones.

Make sure any header that has a corresponding .cc file includes that header
first, to validate that it can stand by itself (as per the style guide). Also
do some simple reformatting (mainly sorting "using" statements, or moving
them to the location prescribed by the style guide).
Use "$(MAKE) -C ..." for running make in subdirectories.

This adds announcements on changes of directory, helping see what happens
better.
Link correctly against a non-installed libcurl.
Have the makefile include local.mk if it exists.

This is to allow setting local variables in a persistent way.
Constify a few methods.
Small reformatting in sqlite_db.cc.
Add requirements.txt to python script
Merge pull request #36 from kovrus/python-script-installation

Add requirements.txt to python script, facilitating easy installation of python pre-requirements using pip.
Add Ruslan Kovalov to the AUTHORS/CONTRIBUTORS files.