Re-factor test fixtures

Concatenation of test fixtures didn't work (see #50), therefore
wavefile_all had to be removed.

This reduces the number of test cases from 157 to 142.
The test coverage, however, stays the same:

    Name          Stmts   Miss  Cover
    ---------------------------------
    pysoundfile     313     65    79%

Most likely, the removed test cases were redundant anyway.

The rest of the test cases should be exactly the same.
Copy test WAV file before opening in 'rw' mode
Remove assert statement at end of if/elif chain

This gives 100% test coverage in the test file itself.
Re-enable file-like objects in 'rw' tests

The right open mode is 'r+w' instead of 'a+w'
Avoid repetition of ['name', 'fd', 'obj']
Shorten annoyingly long fixture names

soundfile -> sf
Use test files with known values, check for exact equality
Replace sf.SoundFile with sf.open in tests

This gives one more line of coverage ...
Make data_mono a column vector
Use read() function instead of read() method in some tests

This adds 9 lines of coverage.
Change file_stereo_w to file_w and file_stereo_rw_new to file_rw_new
Add a test for the write function

This leads to 5 more lines of coverage.
Use read() function instead of method for mono tests
Move all seek() tests into one section
Combine a few trivial test functions to reduce test runtime
Remove superfluous PCM test, add more asserts to FLOAT test
Add test file to check consistent use of default arguments
Use lambdas in addfinalizer() where possible

This the way it was before. A separate local function is only needed for
the case where an exception is expected in the finalizer.
Merge pull request #50 from bastibe/refactor-tests

Re-factor tests