initial stab at exception handling around file operations
Adds hash table write error alert.

Writing can silently fail on certain systems (like mine).  I don't know how to prevent it, but here's a few lines to add in error alerts.
Update Jenkins build status badge
Fix link to virtualenv installation instructions.

Also made link to detailed doc instructions more prominent
move zlib & bzip2 to third-party
sloccount for humans and Jenkins
remaining changes to support third-party/
Merge pull request #427 from ged-lab/feature/sloccount

sloccount Makefile target
move zlib & bzip2 to third-party
sloccount for humans and Jenkins
remaining changes to support third-party/
pip -> pip2 fix
Merge branch 'accaldwell-master'
Move Coverity scan to Makefile; simplify jenkins-build.sh
Merge pull request #421 from ged-lab/feature/simplify-jenkins-build

Move Coverity scan to Makefile; simplify jenkins-build.sh
don't run half the coverage checking if we can't run it all
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