BEANS: fixed converting empty string to attribute value

- When converting string to attribute value convert
  empty string as null, in order to keep equality between
  Oracle and Postgres DB.

  This will also fix case, when attributes are passed in Candidate object.

- Fixed typos in exceptions texts.
Merge pull request #398 from zlamalp/emptyattr

BEANS: fixed converting empty string to attribute value
CLI: fixed creation of perun-cli.tar.gz

- Fixed creation of perun-cli.tar.gz archive from CLI sources, since
  version from November 2013 was copied to AFS on every build
  (because of wrong file name).

- Added execution mark (+x) to CLI commands, which missed it.
Merge pull request #400 from zlamalp/cli

CLI: fixed creation of perun-cli.tar.gz