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
Change NO_ACT so it can be used in addition to other ignores

This results in a more flexible system and is less surprising as it means levels
can be used in the way they normally can in an ignore.

As an example the current approach to NO_ACT provides no way to let HILIGHTS
be shown, with this change /set activity_hide_targets can be recreated with:
  /ignore #channel NO_ACT
  /ignore #channel -except -regexp -pattern . NO_ACT HILIGHTS
(but obviously this can be configured in many more ways if desired).
Don't expand ALL when combined with NEVER/NO_ACT
Remove NO_ACT if we see a user specified hilight

In this path we can't look up ignores again because the print text signal
doesn't know the nick, etc. Instead just show it. The user can use -actcolor %n
or make the hilight more specific if desired.
Add NO_ACT checks for actions and notices
Add NO_ACT checks for DCC messages
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
Make /ignore -replies work with NO_ACT
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