Use a path to ./astromatic/ relative to the current file

Specify the path to the astromatic/ directory relative to the current
file instead of relative to the current directory. In this manner, the
paths to the SExtractor, SCAMP and SWarp configuration files also work
when the code is run from a different working directory.
Add tmp_chdir() context manager
Add mkdir_p() module-level function
Create 'pyraf' directory for cache
Avoid PyRAF "no login.cl found" messages and cache directories
Add LEMON directory to PATH
Use os.path.basename() on sys.argv[0]

It is operating system dependent whether sys.argv[0], the script name,
is a full pathname or not. Use os.path.basename() to make sure that
only the name of the script, 'lemon', is always printed by the help
message, regardless of where it is executed from — in other words,
avoid coming across messages like "See '/home/vterron/lemon/lemon
COMMAND' for more information on a specific command.").
Use os.path.abspath() on __file__

__file__ returns the filename used to locate and load a module. When
the module is found in the current directory, we just get a relative
path, such as 'fitsimage.py'. This makes os.path.dirname() return an
empty string, which then causes methods.tmp_chdir() to raise "OSError:
[Errno 2] No such file or directory:" when it attempts to os.chdir()
to ''. Turn the path into an absolute one so that our code does not
break when fitsimage.py or qphot.py are imported into the interpreter.
Add spaces after commas — as per PEP 8
Remove unused piece of code
Remove unused variables
Print to standard output the path where test images are downloaded
Add missing import — traceback module
Add PassbandTest.read_filter_data_file() static method
Add test data for the Johnson photometric system
Rename UnknownPassbandLetter exception to InvalidPassbandLetter
Add PassbandTest.test_johnson_filters()
Add test data for the Cousins photometric system
Add PassbandTest.test_cousins_filters()
Factor out duplicate code into PassbandTest._test_photometric_system()