Update .gitignore.

Update the top-level .gitignore to fit the generated files I see on a
GNU/Linux system more closely.
Update .gitignore further.

Clarify with a comment that the ignored files are Qt-created, and
exclude only toplevel Makefile.
Merge branch 'namecoinq' into gitignore
Merge pull request #61 from domob1812/gitignore

Update .gitignore.