Developer usability concerns

[drdavella]

There are a few issues that bite new developers (and, speaking from experience, even some more experienced ones) when trying to get started with ASDF and building from source:

• making sure the asdf-standard submodule is initialized and up-to-date
• the fact that it is not possible to import and use asdf from the source directory

It would be nice to address these issues somehow so that new developers are not put off by the project.

[Cadair]

What get's me is the fact that doing pip install -e . doesn't work, i.e. even trival changes to the python source are not reflected in the imported version. This is probably the same as being able to import it from the source directory.

[drdavella]

@Cadair this should be fixed on the latest master. The problem appears to have been the use of use_2to3 in setup.py, which was removed in #409.

[drdavella]

Closing since the major issues here have been addressed. A solution for making sure asdf-standard is always up to date exists in #419, but I am hesitant to merge this due to potential security issues.