Make it installable by PIP

[Matvey-Kuk]

This library needed to be installable by PIP.

"requirements.txt" may look like:

https://github.com/OpenBMP/openbmp-python-api-message/tarball/master#egg=openbmp

Other package's setup.py may look like:

...
dependency_links=[
        'https://github.com/OpenBMP/openbmp-python-api-message/tarball/master#egg=openbmp'
    ],
...

After installation package's main directory becomes directory "where python looks for packages". So after installation now I can import package "api":

from api.parsed.message import Message

because such a folder exists in repository.

I moved library to "openbmp" folder so now one can use it like

from openbmp.api.parsed.message import Message

Reference: https://github.com/pypa/sampleproject (folder "sample" = package name and exists in repo)

[Matvey-Kuk]

@TimEvens is there smth to be fixed in this MR?