new hook - pycparser

Packaging without this hook generates an executable that seems to work.

However, on running, pycparser generates the lextab.py and yacctab.py
modules. This may fail in cases where the executable has no permissions
to write wherever it wants to write: which seems to be the same directory
as the remaining packaged binaries.

Having no knowledge of pycparser itself (I'm using it as a dependency
for cffi which, in turn, is a dependency for cryptography) I'm not sure
if this is the optimal approach.
now using full module names in hiddenimport

Per matysek's suggestion now using 'pycparser.lex' and
'pycparser.yacc' instead of just 'lex' and 'yacc'.

See comment at https://github.com/pyinstaller/pyinstaller/pull/131
add comments explaining the need for pycparser hook

Per matysek's suggestion.
See comment at https://github.com/pyinstaller/pyinstaller/pull/131
adding a test for pycparser hook

pycparser needs two modules -- lextab.py and yacctab.py -- which
it generates at runtime if they cannot be imported. Those modules
are written to the current working directory for which the running
process may not have write permissios, leading to a runtime
exception.

This test checks to see if any files were generated or not. If not,
it means the hook did its job correctly by defining lextab.py and
yacctab.py as hidden imports.
fixed a typo in pycparser hook comments
skip pycparser hook test when pycparser not available
Merge pull request #131 from exvito/hook-pycparser

New hook for pycparser.