initial stab at exception handling around file operations
Merge branch 'master' of github.com:ged-lab/khmer into feature/missing_file_exceptions

Conflicts:
	lib/counting.cc
added in (untested) file error testing in hashtable load
added test for loading truncated counting hash; results in infinite loop
Merge branch 'tests/file_exceptions' into feature/missing_file_exceptions
test for IOError on attempt to load truncated k-mer table
marked source files that need exception handling for reads
shifted eof into general catch block
argparse + optional dropping for 'N's
deleted unused lines, made n's lowercased, and fixed pep8 fomatting
adding argument descriptions and tests for scripts/ graduation
new test file with Ns for test_scripts & added test_fastq-to-fasta in test_scripts
Testing optional argument for higher coverage
Merge pull request #426 from ged-lab/fix/fastq-to-fasta_N

added dropping N counts #395
FASTQ-TO-FASTA output now to stderr, refactoring of test script
FASTQ-TO-FASTA output now to stderr, refactoring of test script
Merge branch 'master' into tests/file_exceptions
Merge branch 'master' into feature/missing_file_exceptions
removed highmem attr from truncated file load test
minor cleanup of _khmermodule: use Py_RETURN_NONE