Fix #6 - Several documentation generation issues

https://github.com/alobbs/libhpack/issues/6

There are several issues with the generation of the documentation:
- *make doc* is broken.
- *make all* does not generate the documentation.
- Docs not be generated if *make all* or *make debug* haven't been run.
- Options *BUILD_DOCS* and *BUILD_TESTS* don't work.

*make doc* issue is easily fixed adding *doc* to *.PHONY*.

*make all* not generating the documentation is fixed by creating a
custom target *doc* in cmake that is added to *BUILD_ALL* if
*BUILD_DOCS* is set.
**Side Effects**: Instead of using different custom targets (*docs* and
*docs2*) we now have only 1 custom target (*doc*) and everyone
generating docs needs to add to its dependencies. That way you don't
need to make changes to the *Makefile* if a new doc generator is added.

To be able to run *make doc* without having run *make all* or *make
debug* earlier I have chosen to check for the existance of
*build/Makefile* instead of adding a dependence to all to avoid having
to compile everything just to generate the documentation.

Parameters *BUILD_DOCS* and *BUILD_TESTS* were not being passed down to
cmake, now they are.
Merge pull request #7 from Akrog/issue#6

Fix #6 - Several documentation generation issues
New test

Adds a new test to cover a conner case
Merge branch 'master' of github.com:alobbs/libhpack
Fix broken integer_decode function
Merge pull request #8 from Akrog/fix_integer

Fix broken integer_decode function
Minor fix

Makefile wasn’t working with GNU Make 3.81. This couple minor changer
should fix it up.
Clean up
New QA test

Initial implementation of a new test to test header strings with
Huffman coded literals
Documentation generation fix

The generation of the documentation takes a long time. It turns down it
wasn’t a good idea to add it to the default Makefile target.
Tweaks debug options
Ignore default CTest directory
Bug fix

Header streams were not fully parsed because the exist clause usually
triggered too early.
Completes QA test

Adds actual checks in the second Huffman headers test suite.
Clean up

Implements assert_store_n_eq() to get rid of the increasing number of
Header Store related tests.
Header table checks

Extends tests to check that the header table is well formed after the
decoding process.
Adds TravisCI and Coverity integration code
Coverity badge