Implementing xmlrpclib.Transport via PipSession

After some attempts with `urllib2` and `requests`, this commit brings a
new proposal for a `xmlrpclib.Transport` implementation via
`PipSession`. Follow changes have been made:

- Reverted `pip/util.py` to move the class `PipXmlrpcTransport` to
`pip/download.py` (near `PipSession`).
- Implemented a new class `PipXmlrpcTransport` using a contained
`PipSession` object.
- Modified the `pip/commands/search.py` to make use of the
`PipXmlrpcTransport` class.