New ESO package

Currently implemented features:
- user portal authentication
- specific instrument query
- direct data retrieval to local files

New dependencies:
- keyring: https://pypi.python.org/pypi/keyring
- lxml: http://lxml.de

Usage:

    >>> from astroquery.eso import Eso
    >>> eso = Eso()
    >>> eso.login("TEST")
    TEST, enter your ESO password:
    
    Authenticating TEST on www.eso.org...
    Authentication failed!
    >>> eso.login("ICONDOR")
    ICONDOR, enter your ESO password:
    
    Authenticating ICONDOR on www.eso.org...
    Authentication successful!
    >>> tbl = eso.query_instrument("amber", target="Sgr A*", max_rows_returned="5")
    >>> tbl.pprint()
    File_upload  target     RA        DEC    data_products.prog_id      data_products.dp_id        ob_id   data_products.dp_cat data_products.dp_type data_products.dp_tech
    ----------- ------- ---------- --------- --------------------- ----------------------------- --------- -------------------- --------------------- ---------------------
                GC_IRS7 266.417056 -29.00614         076.B-0863(A) AMBER.2006-03-14T07:40:03.741 200156177              SCIENCE         FRNSRC,BASE12        INTERFEROMETRY
                GC_IRS7 266.417056 -29.00614         076.B-0863(A) AMBER.2006-03-14T07:40:19.830 200156177              SCIENCE         FRNSRC,BASE12        INTERFEROMETRY
                GC_IRS7 266.417056 -29.00614         076.B-0863(A) AMBER.2006-03-14T07:40:35.374 200156177              SCIENCE         FRNSRC,BASE12        INTERFEROMETRY
                GC_IRS7 266.417056 -29.00614         076.B-0863(A) AMBER.2006-03-14T07:40:50.932 200156177              SCIENCE         FRNSRC,BASE12        INTERFEROMETRY
                GC_IRS7 266.417056 -29.00614         076.B-0863(A) AMBER.2006-03-14T07:41:07.444 200156177              SCIENCE         FRNSRC,BASE12        INTERFEROMETRY
    >>> dp_id = tbl['data_products.dp_id'][0]
    >>> files = eso.data_retrieval([dataproduct_id])
    Downloading AMBER.2006-03-14T07:40:03.741.fits.Z...
    Done!
    
    >>> print(files)
    ['AMBER.2006-03-14T07:40:03.741.fits.Z']