bugfix: gracefully handle Null bytes in matching lines

Usually grep will detect and ignore binary files; but if the binary data
doesn't appear in the first few KB of a file, it might still be detected
as text file, causing GTK to get confused by 0x00 bytes in grep output.
add test file for GNU Grep bug regarding Turkish Unicode characters

The test file contains an upper-case I with a dot (U+0130). When searching
for "a" with case-insensitive search, some GNU Grep versions will omit the
newline between the two lines in this file, which will cause incorrect
output in the plugin's result panel.

Grep 2.10 under Ubuntu 12.04 has this bug; Grep 2.14 from upstream doesn't
have it. I don't see any good way to work around it in this plugin, but I
guess the impact should be quite minor anyway.
bugfix: correctly trim recent-dirs list also if temp. entry was added

Previously, if a temp. entry was added (below separator) to the recent-dirs
dropdown, the list would not always be trimmed to 10 elements, but might
grow larger if many different directories were used for searching.
clarify README file and add installation instructions for Gedit 2