Typo fix

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a .gitignore file

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Generate signature files with appropriate names

For 'gpg --verify <.asc-file>' to work, the .asc files' names have to
match the signed files' names (modulo the extra ".asc" extension, of
course).

Before this change, the plugin would generate, say, a signature
com.example-my-artifact.jar.asc for the file my-artifact-1.2.3.jar,
leaving GPG puzzled against which file it should verify the signature.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Throw a helpful exception when encountering an RSA (sign-only) key

BouncyCastle 1.46 is apparently unable to handle RSA (sign-only) sub
keys while it can handle DSA (sign-only) sub keys just fine.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>