Makefile.am: fixed spacing
autotools: install themes during 'make install'

Default path for themes is ${pkgdatadir}/themes that usualy equals to
/usr/share/profanity/themes. This patch also introduces new option
--with-themes[=PATH] that specifies installation path. This option can
be used for cancel of installation at all: --without-themes.

References: issue #323
config/theme: search for system-wide themes

Search for system-wide themes (usually /usr/share/profanity/themes) if
not found in user's home directory.

Not tested yet.
autotools: added EXTRA_DIST

Now sources can be distributed with `make dist-bzip2` command, so no
need to make tarball manually. Current patch takes into account the
status (release or development) and puts .git/HEAD, .git/index to
tarball if necessary.

Unfortunately, current scripts don't add any information about
development status to the version string. Therefore,
profanity-0.5.0.tar.bz2 is got regardless of development or release
status.
Merge remote-tracking branch 'pasis/iss323'
Fixed makefile to include headers in test sources