Fix typo in README (SNMP contents, eh?...)
API change: swap "oidstr" and "initval" arguments for scalar variables

The function signature for create_vartype_class() has changed and as such the constructor
method signature for all classes using the @VarTypeClass decorator, that is, all classes
representing SNMP scalar variables, eg. Integer32(), Unsigned32() etc. "initval" now
comes first so that in scenarios where we just a SNMP variable without the intent of
registering it we can simply write

  agent.Integer32(20)

instead of

  agent.Integer32(initval=20)

which, of course, implies "oidstr" == None. The former construct is of much more use when
being interpreted as "initval" instead of "oidstr" as in

  agent.Integer32("EXAMPLE-MIB::exampleInteger32")

which continues requiring an "oidstr=" prefix to work.

This means that if you have already written agents using python-netsnmpagent and you did
not follow the style in example_agent.py with explicit "initval=" and "oidstr=" qualifiers,
you WILL have to adapt your code.
Replace hardcoded version number with latest Git tag

With this change, the version number is no longer hard-coded inside setup.py. Instead, the
version number is derived from the latest git tag using "git describe". The Makefile (which
is now more than a pure convenience Makefile) will generate setup.py from setup.py.in
(which has been renamed accordingly), so that a source distribution (.tar.gz) will have
the correct version number inside.

As a special rule, whenever the Git checkout is not at a tag, "git describe" will generate
a string similar to "0.2-21-g0e10fca", indicating that there have been 21 commits since the
last tag "0.2" and the latest commit is g0e10fca. We translate this to "0.2_next21_g0e10fca"
so a.) RPM does not complain about the dashes (it doesn't allow them in its "Version" field)
and b.) the "next" indicates that this is a forthcoming version.
Add MANIFEST.in file so source distributions include necessary files

distutils by default expects LICENSE.txt, not LICENSE. It also needs to be
explicitly told about our example agent and associated files.
Automatically generate ChangeLog from git commits

We now have a ChangeLog automatically generated from the Git history, so users who do not
follow the proceedings in the Git repository itself can see what has changed between each
version. The ChangeLog is built whenever a source distribution archive is generated. It
can also be explicitly built by using "make ChangeLog".