Support markers in requirements

It's now possible to specify requirements markers in requirements.
Examples::

    futures; python_version < '2.7'
    mock; python_version < '3.3'
    nose
    ordereddict; python_version < '2.7'
    unittest2; python_version < '2.7'

The separator is "; ". For convinience, ";" alone is also supported, but
no in URLs. The ";" character is a legit and common character in an URL.
Example of valid URL without markers::

    http://foo.com/?p=bar.git;a=snapshot;h=v0.1;sf=tgz

Example of URL with markers::

    http://foo.com/?p=bar.git;a=snapshot;h=v0.1;sf=tgz; python_version < '3.3'