Issue #11: Script to auto generate huffman tables

https://github.com/alobbs/libhpack/issues/11

Currently, the Huffman tables are hardcoded in the source code.
It'd be good to have a script to parse the HPACK spec document and
generate them from there.

huffman-gen.py Generates huffman tables for HPAC encoding and decoding
based on the draft document.

It uses a template, default is huffman_tables.tpl, to render the final
output.

We can choose our source between those already known to the script:
	- EC_HTML: Editor's copy in HTML
	- EC_XML: Editor's copy in XML
	- EC_TXT: Editor's copy in TXT
	- WGD_HTML: Less recent, more official Working Group Draft
Or specify our own source file.

It tries not to modify the output file, huffman_tables.c by default, if
possible. That way it will not trigger a recompile of the source code.

Tries to do as little work as possible, so if it detects that the source
file hasn't changed (etag, date, size in headers) from the last time it
will consider the output file to be up to date and stop there. Won't
even bother downloading the source file.

If you change sources it will not be able to tell if the file
has been updated that way.

If he doesn't think it's up to date it will generate the arrays and
compare them with the current contents of the output file. If the arrays
are the same he doesn't touch the output file.

Only when they don't match he will write the output file.
Script can be forced to always generate output file.

**Side Effects**:
To be able to tell if the file has been updated we have added a line in
huffman_tables.c file (second line).

New file created huffman_tables.tpl
Issue #11: Include huff_gen script in cmake

https://github.com/alobbs/libhpack/issues/11

huffman-gen.py needs to be executed when building hpack libray.

Add the custom command needed for huffman-gen.py to run when building
hpack library.
Issue #11: Removed comment from last commit

https://github.com/alobbs/libhpack/issues/11

In last commit an unnecesary comment was added to libhpack/CmakeList.txt
This comment has been removed
Merge pull request #19 from Akrog/huff_gen

Huffman coding and decoding tables are regenerated from the spec document now.