Initial implementation of 256 colour support for Irssi

This patch implements some 256 colour support for Irssi up from the
previous 16 colours. Initial parsing of the %x/%X format codes is
implemented and the parser accounts in advances the char* for
that.

The colour attributes are widened from 4 to 8 bit. The colour protocol
is changed to a new format. Some pointers to remaining work are
written in the comment in textbuffer.h.

Note that Irssi already does support requesting 256 colours from the
terminal in the original source code, so this part did not have to be
touched.
Make it more obvious if -pattern wasn't provided to a regexp /ignore
Warn with error if regexp ignore fails to parse
Suggest using ^ so this doesn't pop-up a NickServ window
Finish 256 colour support for Irssi

256 colour patch is cleaned up and the remaining cases are made work,
this includes especially Theme support, which was not implemented
before. Changes not related to colours were reverted again, making a
review of the two patches against master easier to follow.

As a byproduct of the Hex-colour code parser, the 24bit colours are
also implemented. Actually using them in the terminal is guarded by a
compile time switch (as well as a run time switch), as it breaks the
existing colour protocol and requires additional storage.

To make a seamless usage, down-conversion is provided for 8 and 16
colours.

Diverging from Tom's approach, the colour protocol is reverted back to
the original one. Unfortunately, the changes required in the Theme
engine will break the API.

For more details, please refer to the patch documentation at either
http://irssi-docs.wikispaces.com/Notes-256-Colour or
https://github.com/shabble/irssi-docs/wiki/Notes-256-Colour
point out more modules in README
Remove superfluous entry from EXTRA_DIST

curses.m4 is already included by automake as it's an m4 depdendency.
propagate flags for automake initialization
Refer to correct name of README.md

fcb67831f2759fb renamed README -> README.md but didn't update
EXTRA_DIST, breaking 'make dist'.
Improve docs of special_vars wrt. to escaping rules in /alias
Merge pull request #76 from ailin-nemui/improve-special-vars

Improve docs of special_vars wrt. to escaping rules in /alias
Merge pull request #73 from ailin-nemui/readme-other-modules

Point out more modules in README
Merge pull request #71 from dgl/nickserv-doc

Suggest using ^ so this doesn't pop-up a NickServ window
Removed docs/help from the .gitignore
Merge branch 'master' of https://github.com/irssi/irssi
Aligned the /NETWORK documentation with docs/manual.txt
Added a default HIGHLIGHT and DEHIGHLIGHT alias
Merge pull request #74 from falconindy/master

Fix make dist
Merge pull request #72 from dgl/regexp-error

Warn with error if regexp ignore fails to parse
Merge pull request #70 from dgl/regexp-pattern-missing

Make it more obvious if -pattern wasn't provided to a regexp /ignore